For most people, 32 bit is just fine, for now. However, as technology advances, the 32 bit OS is a bottleneck when it comes to memory addressing and processing capability. Today there are more and more computers starting to reach the point where the bottleneck becomes an issue. For instance, many computers come with 4 GB of memory already installed. This is the maximum that a 32 bit OS can address, meaning that if you want to upgrade that computer's memory ever, you will need a 64 bit OS.
The demand for faster computers always catches up to what's on the market.