Vegetable starch, experience points, merchants where you want them, loot from combats, materials scavenged by settlers, gun emplacements, and purified water, in that order.
All weapon and armor mods except the basic ones require adhesive. If you have a thriving settlement network, vegetable starch will provide all the adhesive you ever need. Otherwise, it's a significant bottleneck. You can't really scavenge enough, and buying more from vendors only somewhat closes the gap.
This is particularly true if you enjoy using explosives. Making grenades and mines require adhesive, and since they're consumables, you can't really afford to make them if you don't have that limitless supply of vegetable starch.
Every time you build a settlement part you gain experience. That's a fairly substantial benefit - I know I've gained several full levels just as a side effect of building up housing and defenses for settlements. Doing this primarily uses up wood and steel, which are plentiful. Defenses use additional materials, and there's some small overlap with other crafting needs, but it's not a big deal.
It's extremely convenient to have a set of merchants set up near your primary stash of loot. You can pop a Grape Mentat, load up, and sell much of it all at once. No need to keep your load down to fast travel limits. You also get to buy ammunition whenever you feel yourself particularly short of a type. There are ways of getting some of this effect without settlements, but settlements make it easier.
Those "defend this settlement" missions can seem annoying, but it means you're fighting a large group of enemies with LOTS of fire support from turrets and settlers. This yields experience and loot. Sometimes a little, sometimes a lot.
In the same vein, there are settlements where you can set up a wall of turrets and then lead hostiles into them, because there's an enemy-held location very close. Not so important in the late game, but it can make clearing tough spots much easier. Finch Farm and County Crossing come to mind.
Eventually you do get those artillery strikes, though I never liked those much. They're free and do a ridiculous amount of damage, like carrying around a delayed-launch Fat Man.
Less visible is the materials gathered by unemployed settlers, and settlers assigned to scavenging stations. This income is relatively small - 1 or 2 units per settler per day, of any possible crafting material - but it adds up when your population gets large across all your settlements.
Finally, those Industrial Water Purifiers produce a ridiculous amount of Purified Water. Which you can sell, for example.