There are roughly 4 categories of applications for clients:
Sequencer applications, MIDI players, MOD players
Device drivers, low level drivers for sound cards, OSS emulation
Event processors like MIDI THRU, instrument mapping
Service providing clients: handling synchronization and tempo, patch manager etc.
The first category is the one that will be used by most application programmers. This is comparable to the OSS /dev/sequencer or /dev/music interface with the big difference that multiple clients are allowed.
The other categories are more specific to the ALSA sequencer, and are needed (especially the device drivers) to get any sound out of it.