Best guesses according to the internets, seems to be a hold over from the New Year being celebrated on April 1st, to celebrate the arrival of spring. However, since the New Year was switched to January, April 1st became obsolete. Historically, one could trick people into arriving at a New Year's celebration 3 months late.
Now a days, the "purpose" is to allow professional organizations, who normally have to be straight-laced and responsible, to have a little fun. Real life pranking seems to be minimal, but websites and news outlets get to let their hair down and go a little crazy for one day. Take a look at the ENWorld battle, and once you know it's a joke, you can see that the Mods were having a lot of fun.
Normally they have to be the reliable ones, watching over the rest of us. For one day, they get to be the ones having all the fun.