When to use it: you have a big network and/or perform SNMP commands for a number of OIDs and want to figure out which commands, devices and OIDs fail, what OIDs are inactive, which commands take longer to respond etc. Watching it in real time is unproductive as the events happen too fast. You can collect data into a log, but if it's too big, looking through it is hard if you're not searching for one particular error. In addition, if you generate more logs, you have to keep looking through them.
With the Netplayback application, you can load your SNMP log, and either display it in a sorted request-response fashion, or "replay" the entire event sequence in a movie-like format. You can change the speed of this movie, pause this movie at any time, forward or rewind it or automatically stop it on an error. All these functionality is available for free. In addition to this, you can filter the replay be router, OID and data type. This functionality is available with a proprietary license, which is available for purchase.