An example would be best.
Let's assume you've got a track with a fairly detailed envelope on it with lots of nodes & therefore level changes.
You decide the whole track needs to be shifted up or down by a few dB, so you have a choice.
- Painstakingly edit every node by the same amount
- Adjust the level via one of the many points in the audio chain (eg, track input gain but this isn't foolproof as the level fed to any subsequent plugins will also change which will mess up compressor thresholds and suchlike
- Pop into offset mode, change your track fader, then come out of offset mode.
The only drawback is that the displayed level will be different to that shown on your meters, but provided you know about it, it's not a problem.