If you don't know what it should be set at, usually default is a good choice
But yeah, overall it's basically a way to change how long players develop or age for, rather than how fast. Ie. One of the complaints forever is "man it sucks that my guys drop off a cliff at 30". Yes, you can slow the aging speed to prevent that but then people are like "why are all these 42 year Olds still playing well?". So if that happens to you, you can set the aging target older, so that players won't start declining until 32 or 33 or 34, but can also separately set the speed if you do want some of them to fall off a cliff.
Same with development. If you feel the base age that players develop until is too soon or too far, you can tweak that.
And true, you can more or less "cancel out" the changes in some ways, or do different changes and have it effectively work the same way. Some of this is also just how you think - do you want to "speed up development" or "have players reach their peak sooner". Different ways to slice a melon.