What can SNMP be useful for in a small network?

We currently use Cacti with SNMP to monitor our servers and network, and it is invaluable.

With SNMP, we get automatic host availability notification, so that if a server or firewall fails, it doesn't go unnoticed.

It allows us historical tracking of both performance, and in the case of servers, storage utilization. We can then identify potential upgrades, or re-allocation of resources when necessary.

It also gives us the ability to make pretty maps of our network utilization, to identify when and where there is a problem (using Cacti and Weathermap):

The more knowledge you have about your infrastructure, the better decisions you can make.

Sure! Smaller networks often have servers, switches, and routers just like bigger ones. Combined with the right tools, SNMP can be very useful for trending performance for the long term as well as alerting on certain conditions.

Take a look at tools like Cacti and OpenNMS to see what possibilities are available to you.