There must be a reason for the sustained release.
There is, but a finance major such as myself could likely explain it at least as well as any pharmacist, seeing how it has basically nothing to do with pharmacology.
GSK (big evil drug company) wanted to milk their cash cow for as long as possible, thus multiple reincarnations of Wellbutrin.
1. Wellbutrin: take every 8 hours
The patent is near expiration, so go to step 2:
2. Wellbutrin SR: take every 12 hours
Damn, that patent is near expiration again, so go to step 3:
3. Wellbutrin XL: take every 24 hours
Damn, we've run out of steps and the bottom line is sinking!!! Well, had a good run, staying under patent for 20+ years with this silly, though, quite common game where the FDA deems slapping on a time release coating to be a "new" drug.