It is very simple.
1. If you intend to regularly use your computer to plot routes but do not intend to use them on your device straight away, then install the maps into BaseCamp. Examples of when this might be a good thing to do are:
i. You create lots of routes for other people's holidays, which you'll send to them by email
ii. You plan your two week trip away in June but create the routes for it in December
For either of these you do not need to attach your GPS device all the time, so it's easiest to have the maps fully installed. Please though note that, as and when you DO attach your device it WILL still take a minute or so to drag the installed maps across, even though they are already on your computer.
2. If you intend to only use your computer occasionally and do not want to have waited the two or so hours that it would have taken for you to have installed the maps onto your computer, then don't install them.
As I use my computer regularly and do not want or need to attach my GPS device each time, option one is best. I update my maps overnight, so the two to three hours (or maybe longer) it takes to update my computer AND my device in one go does not inconvenience me.
To you, who cannot wait the extra - possibly overnight - time it takes to install maps on your computer, or does not need the maps permanently installed on your computer in the first place, option two is probably best. The time (it is only a minute or so) that it takes for your computer to drag your device's maps across will happen every time you attach your device whether you have pre-installed the maps onto your computer or not.