Other than being able to do things twice 'fresh', not really. In GT5 the extra accounts were mostly for gifting purposes but that is one of many features removed for GT6.
Pointless in GT6. No gifting, not necessary for GTP_xxxxxxxx signups.
Was quite useful for GT5. All of the redundant, dummy PSN accounts may be the reason gifting was abolished in GT6. It certainly had to be a consideration.
There's no reason to make another account unless the Community features that were in GT5 are implemented in GT6. I do have a second account though because I'm trying to complete the game using only unmodified cars, which I'm finding quite challenging.
I use a second account so other people can use it without messing things up on my main account... and also because I used the 1st versions vision Gran Turismo car glitch A LOT, I use it as my tester so as I can know what cars to get and what tunes to use so as will not waste money or time on my 2nd account... not to mention staring again "fresh" I may pick up some style or technique that I missed the 1st time... I try that on most of my major games...