Let's say you have a number of systems on your scanner, as I do in the Dallas-Fort Worth area.
You have the scanner going, covering whatever you have programmed. Nothing much happening out of the ordinary......
Some sort of large incident happens. Multiple alarm structure fire, maybe a vehicle crash with multiple injuries & also a haz-mat response for a chemical spill, the possibilities are endless.
If everything is being conducted on one frequency, then, sure, press "Hold" or "Manual" and stay on that frequency. But if it's a large incident, then it's likely several channels will be in use; depending on the agencies involved, that may be all on one system (especially in larger cities). So, you would use system hold to focus on that one system (and/or agency) to listen to the activity, rather than trying to turn off (or lock out) the other channels/frequencies/banks/scanlists/systems (depends on whatever programming scheme your scanner uses). Much quicker to do with "system hold", and much easier to return to 'normal operation' after whatever you were focused on is resolved.