Hey, Charles; welcome!
The short answer is, plunge bases are for handheld usage and fixed bases are for upside down use in tables, as well as handheld use.
OK, now for the longer answer. Some plunge models like the Bosch 1619EVS come equipped with built in spring lockouts which allow you to run the height up and down without fighting against the column springs. As bob explained others have through-the-base adjustment mechanisms which allow for adjusting height without lifting the router out of the table.
Other brands and models come as package deals with both fixed and plunge bases, for example the Bosch 1617EVSPK...you use the fixed base in your table, and simply release the router from the base and slip it into the plunge base.
You shouldn't ever have to remove the base plate to adjust ht. and if you do, you need to be precise in remounting it exactly centred! There's an inexpensive little accessory which aligns the plate exactly.
This is critical for some operations.
In summary then, ideally you have either two or more routers as described, or a package which does both jobs.
Fair warning; woodworking is a very satisfying craft but it ain't cheap.
Ask away; the members here love to help newbies get off to a safe and rewarding start. You'll often hear members say the only stupid question is the one not asked.
Oh! Please be REALLY careful...routers can bite!!!