The SNMP by MIB monitor is used when you need to build the counter tree for the monitor , while if you know the specific OID that you want to monitor you could just put it in a regular SNMP monitor.
The following extract might ilustrate this better :
SiteScope SNMP usage In SiteScope, the SNMP monitors simply do GET request to port 161 of an SNMP enabled device, and the device being monitored responds with a GET-RESPONSE message containing the information SiteScope has requested (or an error message for requests that fail).
In SiteScope a MIB is not necessary for the SNMP monitor. With the SNMP monitor, only the OID is necessary. Once the OID is specified, then the MIB is not needed. The only use of the MIB would be to figure out the specific OID for whatever needs to be monitored on the remote SNMP device. There is another monitor in SiteScope called "SNMP by MIB" that will use a MIB to build out the counter tree for you in SiteScope, that monitor has it's own tech note.