"why not use the same spring?"
Springs are a wear part too. I reused the original springs when I upgraded some black and green milspec followers to Magpul, and they've worked fine. If a magazine has feeding trouble with good feed lips and a good follower, replacing the spring seems like the next part to replace.
As far as the different springs for 20 and 30 round magazines, the 20 round mag is shorter. You can cut a few coils off a 30 round spring then bend to fit if you need to make one work.
As previously mentioned, you can modify a Magpul follower to work in a 20 round magazine.
Jimro