I'm assuming you're talking about removing (i.e. uninstalling) "old" maps from your PC?
If so then then unless you have a specific reason to need the old cartography (perhaps a second navigation device that you haven't updated) then it's sensible to uninstall old maps as they just eat up disk space. Remember also that if you are using Garmin Express to install the maps then the initial download is also stored on your disk and is consuming space. See this post on the Garmin Forums for details.
The only time I've ever found it helpful to have an old copy of the cartography available is when I want to precisely reprise a particularly intricate trip using the current maps, and need to be absolutely certain that the recalculated version using the new map data is identical to that I created with the old cartography.
Thanks st,for that detailed information,and the link.
I find it's vital to be using the same map version on pc, that's on device when you are transferring routes /waypoints ect,
to avoid problems on the road.