Overview
Writing to memory is slower than executing an instruction. A store buffer allows the CPU to 'finish' a store instruction and move on to the next one immediately, while the actual write happens in the background.
Store-to-Load Forwarding
If a subsequent 'load' instruction needs the data that was just 'stored' but hasn't reached the cache yet, the LSU can 'forward' the data directly from the store buffer to the load instruction, avoiding a slow cache access.
Importance
Critical for hiding the latency of memory writes and maintaining high instruction throughput in out-of-order processors.