There are unique weapons–the ones in the crocodile leather cases–you can find in certain locations. Other weapons you get from FND chests, rewards, or purchase, and as @stusser says, you upgrade your guns using mats you get from loot.
For me the draw is, yeah, the combat itself, as well as advancing the story (such as it is). The bad guys get meaner as you go farther in the story, and you get directed to different areas of the country. It’s about on par with FC 5; I never found the skills/perks that compelling, and here you can carry four guns from the get-go at least.