Personalization mostly, I mean how many Squier Jags and Jazzes are there in the world? Lots! So modding mine serves to separate them from the herd.
But in the case of the Jag, that active bass circuit and the pups HAD TO GO. I tried the Fender Jags that were available, but none called to me, the Squier did, except for that active circuit. I also really wanted to try the EMG Geezer passives so that was a win-win for me.
I've got a cheap chinese knock off J that I'm going to convert to P/J, why? Because it was really cheap, I have a main and backup player already so other than being handy for woodshedding, this bass has no real function or purpose, so a little amateur luthery seems in order!