No alignment isn’t the only reason for control arms. They also can give you more suspension travel when paired with the proper coilover/shock. Aftermarket control arms are also usually much stronger than the stock ones and can take more abuse. There’s also the option of if you want heim joints or grease-able bushings and ball joint style or uniball.
As far as which is more important to get first, I would say the upper control arms. You’ll be able to get a better alignment right off the bat if you’re running bigger tires. Most aftermarket upper arms will also come with some built in caster and camber adjustment as well compared to stock.
I kind of touched on the purpose of uppers and lowers already but they don’t do the same thing exactly. Most people aren’t going to need after market lowers honestly. Unless you’re going long travel or run your rig off road a lot or through rough terrain a lot, you probably won’t need them. Again, compared to stock they’re much beefier and sturdy and I’ve heard they do help stiffen up the front end a bit.
Now I’m no expert either and if you want professional knowledge I encourage you to reach out to the companies that sell the control arms and pick their brains. Camburg racing is personally my favorite. There’s others in this group that have more knowledge than me as well but this is my knowledge at least. Hope it helped a bit.