While the three American League divisional races seem to be more or less settled -- Texas won the AL West on Friday, while Boston and Cleveland each have leads of at least 5 1/2 games -- that doesn't mean that those clubs no longer have anything to play for. Since all three teams have either 63 or 64 losses, the race for home-field advantage in the playoffs is still on.
Perhaps as importantly, there's still a big open question about which two of those division leaders have to play one another, and which gets to welcome the winner of the AL Wild Card Game. Depending on how the final week of the season plays out, it could be only a series of tiebreakers that determines who goes on the road to face the No. 2 seed and who stays home to face the fourth or fifth best team in the league.
Speaking of those tiebreakers, they break down like this. Both the Rangers and Red Sox won the season series against the Indians, so they would get the No. 1 seed if they finish with the same record as Cleveland. And if the Rangers and Red Sox tie, the Rangers would get the top seed because even though they spilt the season series, they have a better record against the American League West than the Red Sox do against the AL East (that could change, though).
And if there're is a three-way tie, the Rangers get the No. 1 seed because they have the best overall record (8-5) against the other two. The Red Sox were 7-5 against the pair and would get the No. 2 seed, followed by the Indians, who were 4-9.
Obviously, all three clubs want to face the Wild Card and start off at home, so perhaps it's less about finishing first than it is about not finishing third, therefore being the only division winner without home field in the first round. Let's look at all three teams and see why, beyond the obvious reasons, they'd be well-served to keep going for it despite having a playoff spot wrapped up.

: Texas: Because one-run magic works a lot better in the bottom of the ninth.
Without question, the story of the Rangers' season is "success in one-run games," which you might also call, depending on your perspective, "lucky," or "clutch." It was clear as far back as June that the Rangers were winning by coming up big in the most important situations; since they've now managed to keep it up all year, their 36-11 record in one-run games is now borderline historic. (This, despite the fact that they've outscored their opponents by just 14 runs, about the same as the 73-81 Colorado Rockies.)
We can argue all winter long about whether that kind of success is sustainable beyond a single year, but if that's how this team has managed to challenge for the best record in the league, then so be it. It's just that if you're going to rely so heavily on one-run victories, then it's understandably an advantage to be at home -- not so much for the friendly fans or the home cooking as the simple opportunity for walkoff victories.
Texas is 41-38 on the road, but 50-25 at home. Part of that is because when you break down the Rangers' performance in one-run games, you'll find a good mark of 15-8 on the road, and an absurd mark of 21-3 at home. It's a lot easier to make that magic happen on your home field.

: Cleveland: Because they have baseball's biggest home/road hitting splits.
While rumors of Cleveland's demise in the wake of injuries to Carlos Carrasco, Danny Salazar, and Yan Gomes were sorely premature, it can't be ignored that this is a team that's going to head into October at something less than full strength. (Don't forget that Michael Brantley was unable to fully return from his shoulder injury, and fellow outfielder Abraham Almonte is ineligible for the postseason due to an earlier suspension.)
That being the case, the last thing Cleveland needs is to be at a disadvantage in other ways, and being on the road might be doing just that. It's not that a .500 (37-37) mark away from Cleveland is bad, necessarily, it's just that their 53-26 record at home is the best in the American League -- and a huge part of that is because of the offense.
Consider this. Using Weighted Runs Created Plus, an all-inclusive park adjusted metric that allows teams to easily be compared to one another (100 is set as "league average," so 110, for example, is read as "10 percentage points better than average), Cleveland's 121 is the second-best in baseball, behind only Boston. (Texas is seventh. Everyone likes being at home.)
But when you look at it on the road, well, the Red Sox are still second. Cleveland, however, falls to 27th. The only teams they outhit on the road are the Reds, Royals, and Padres, three teams who will not be joining them in October. Francisco Lindor, for example, hits .350/.404/.502 at home, but just .259/.303/.366 on the road. Mike Napoli is .280/.391/.576 at home, and .204/.276/.383 away. Single-season home/road splits aren't usually predictive of the future, of course. Still, the depleted Indians don't need any more bad karma.

: Boston: Because you want the other teams to have to go on the road.
If this feels like a cop-out, well, perhaps it is, because the Red Sox are so well-built and playing so well that gaining home field seems less about empowering themselves and more about diminishing their opponents. Remember a second ago when we said they were the second-best hitting team at home, and also the second-best hitting team on the road? They also have a nearly identical record at home (46-32) as they do on the road (44-32, after beating the Rays on Friday).
Now, it is true that the pitching has been better on the road (3.67 ERA entering Friday) than they've been at Fenway Park (4.35), but that's hardly reason to think the Red Sox want anything than to start the ALDS at home in the Bay State. They want to welcome the Rangers or Indians, or ideally the Wild Card team -- perhaps preferably the Orioles, since Baltimore struggles badly against lefty pitching (.309 on-base percentage is the third-lowest in baseball) while Boston could throw lefties David Price, Eduardo Rodriguez, and Drew Pomeranz, in addition to AL Cy Young Award candidate Rick Porcello.  
The Red Sox have won nine in a row, as the much-maligned pitching staff has come around to support baseball's best offense. Sure, the Red Sox want to be at home. Unfortunately for the rest of the AL, it may not matter.