This subject comes up often and it is a valid question. I have more scanners than most, 4 mounted in my truck (along with 3 two-ways) and 18 currently in my home office, as well as several at work and a dozen handhelds.
Most of us with many scanners devote many to specific purposes such as data collection, logging, live feeds or specific channels. I have 6 that I use to monitor and record fire toneouts. I also have a set of 4 radios that are used for data collection on local trunked systems, one I use for local aviation and others used for more generalized operations.
In the car I have one dedicated to CloseCall operations, one for our local statewide trunked system and another for rail and aviation. The last is used sometimes for mobile data collection (scouting out trunked systems etc.) or searching out new channels.
While people like me are the radical end of the spectrum, there are occasional purposes less intensive than mine that call for multiple radios. Perhaps one wants to monitor different services and dedicate radios so that there is less chance that communications would be missed due to activity.