It will get you earlier access to goodies that come with the expansion. (The free boost that’s been a part of each expansion since Warlords would probably be the big one.) For Legion, you got to play Demon Hunter about a week early, and as Anacrussys mentioned, BfA got you early access to the allied races.
Keep in mind that if they bundle the new expansion with BfA when pre-orders start, you’ll likely lose out on the separate BfA boost if you wait. (Whereas if you buy BfA and then the expansion seperately, you’ll get both the BfA boost and any boost they end up offering with the new expansion.) If you want to wait for anything, wait for their Black Friday sale. There’s a good chance you can get BfA at a discount then.
We’ll all know a lot more after Blizzcon this weekend, so at the very least, I’d probably hold off on any purchases until then.