There are four main rationales for 64 bit cpus as far as I can see:
1. To break through the current RAM access limitation (which is about 4 Gb I think). This is actually a very good reason considering the explosion in RAM usage. I can remember when 16K was an expansion pack! My desktop has 1 gig of RAM and its maxed out. RAM is faster than magnetic disk so keeping more stuff in RAM makes things go faster.....
2. Improvements in processing efficiency. This is not so obvious to me due to the need to reprogram applications to make the best use of 64 bit OSs. When Apple moved from 24 bit to 32 bit about 8 years ago (?) there was a fair amount of nashing of teeth as software authors hadn't obeyed rules about memory addressing. That wasn't even a true architectural change. After a period of catch-up and learning of new coding tricks that optimize 64 bit processors, there should be significant speed advantages.
3. To get us back onto the upgrade cycle. These poor computer companies need to feed their jets. We seem to be reasonably content with only throwing out a perfectly good computer every 4 years now, rather than every 3. This unhealthy practice has to stop people! You shall buy new. What do you think your computer is? A car? A comsumer electronics device? It's meant to be disposible.
4. Marketing (same as 3).
So, when can I get one!!!!!!!