64 bit allows you to access all of the RAM that your computer has available. With 32 bit, you are generally limited to 4 GB of RAM access.
If you are using RAM hungry software like LA Scoring Strings, Omnisphere, Altiverb, you can hit your RAM limit pretty quickly.
If you have a large template and you’re a running a lot of plug-ins
64 bit it essential.
As small session for me is at least around 40 tracks, with plug-ns on every track. I would hit the wall in no time at 32 bit, even though I use two Mac Pros connected by VEPRO.
The only downside to going 64 bit is that your 32 bit plugs won’t work unless your DAW can handle both simultaneously.
I highly recommend VEPRO, because it hosts outside your DAW.
_Michael