This is a very basic doubt that I have. I am not an IT or CS guy so please try to explain in simple language.
Now why I am asking that question is that because we can run 32 bit apps in 64 & 32 bit OS's. AFAIK the data types for 64 bit take double amount of memory than 32 bit apps. Also 64 bit apps can only run on 64 bit OS's.
Then why take the trouble of building 64 bit applications?
Perhaps thats why Firefox is available only in 32 bit???
Sorry if this question is not on par with the standards of SO, but I just cant control stop thinking about the same.
Thank You.

UPDATE: Somehow there seems to be a confusion.
I did not mean to question why we need a 64 bit architecture machine.
I know that 32 bit machines can only use 4GB RAM & 64 bit machines have much higher limit.
**I was questioning why we need to build 64 bit Applications!**