Compared to amd64, x32 offers significant memory savings, often on the order of 30%, and modest efficiency gains.
Compared to i386, there's no memory use difference, but speed increases are more pronounced, especially in code that's under register pressure, operates on 64-bit or floating-point variables. It also avoids i386's penalty for PIC code, where EBX is essentially reserved for the Global Offset Table (GOT).
There are three principal use cases:
vserver hosting (memory bound)
netbooks/tablets (low memory, want performance)
scientific tasks (want every % of performance)