There is an adage in baseball that "sinkerball pitchers improve as the game goes", that in particular the sink improves - so sinkerball pitchers tend to give up a lot of fly balls in the first few innings but then get more sinking action later on, leading to sinkers actually sinking and yielding ground balls (or swing-and-miss). (I can't immediately find this online, but it's a common adage from sportscasters; Steve Stone just mentioned it during the White Sox - Twins game on 7/7/2021).
If this is true - and I'm open to statistics otherwise - why? If it's because being tired helps, why don't they just throw a few dozen extra pitches during warmup at full speed? If it's something else... what?
Well to be honest, a lot of sinkerballers would "doctor the ball" (adding foreign substances to the ball [Vaseline, spit]) to increase downward action. Baseballs that spin faster (spin backwards in particular) give the illusion of the ball "rising" (not physically possible if throwing overhand) because the back spin is helping it hold its plane longer than a ball with less spin, or ball with Vaseline. The sinkerballer is actually playing off of the opposite phenomenon (although sinkerballs do actually sink). It's all about deception and the more different pitches I've shown you the harder it will be to anticipate. (A sinker isn't a swing and miss pitch; it's to induce poor contact.)