Why should you care about your settlements? [censored] vegetable starch, man!
Really, that's the biggest drive for me; adhesive is incredibly hard to come by for weapon and armor modification if you don't have adhesive. Vegetable starch will deliver adhesive in spades if you know what you are doing.
What you need are Mutafruit trees, Tato plants, Corn and a source of water, in excess of the amount of villagers that you have, which in turn means that you need to build up defenses and keep them happy.
I did that with the Starlight Drive-In. I did install some mods to increase the capacity of settlements; there's a limit to the amount of objects you can place there and the amount of settlers you can have in one place is limited by your charisma. Both are stupid in my opinion, since settlers are by default happier if there's more of them, so why would you need to be a charismatic leader to convince them to do what already makes them happy? So I got the mods and now I'm making a miniature fort.
In short; settlements are a convenience for weapon and armor modding at first, then become a motivator of their own for you to scavenge for more supplies to keep your settlers safe as they drudge corn fields for you. You'll need the leader perk, though; it's basically impossible to do without it, since the supply lines link all your workshops together.