Kynxah
Let's pretend 100 users join each day, and they're each given 25 cash. 100 x 25 = 2500, which equals a $25 cash card. That means each day Gaia is 'giving out' $25. Now think about how many people really join each day, including new users, mules, trolls, and bots.
Hundreds of potential Cash buyers. :3
When Gaia first started the cash thing, they gave out 25 cash to users. This was with the intent of saying "HEY LOOK! You can get cool, exclusive stuff from this one store with this new kind of currency!" And for a lot of people, I assume it worked to make them buy more cash.
Giving out a LITTLE free cash upon joining, like 25--enough for one fish or one small cash item--would likely encourage new users to buy cash because "OMG I WANT THE EXPENSIVE ONES TOO!" sort of thing.
But yeah, giving out 25 cents doesn't seem like a lot until you realize just how many users are on this site.
EDIT
I think I just totally misread your post. ><