Event-driven buffering generally allows you to have a smaller buffer for lower latency. That's because it tells you (via BASSWASAPI which calls your WASAPIPROC function) when more data is needed, which means your processing can be in sync with it.
Note BASS uses event-driven shared mode WASAPI itself these days, so you won't need BASSWASAPI for that. The main reason to use BASSWASAPI would be for exclusive mode.
