If you're familiar at all with 1960's music, you'll recognize their work, it permeates popular music.
The artists themselves were often poor musicians and could not play their own music very well.
After watching the movie, I certainly concluded that the record label added a LOT of value.
So, in that case the record label was "adding value" but the Monkees or the Byrds or whoever didn't really receive that value. They didn't get paid much anyway, so they would have rather made less-polished records that were more authentically their own.
The best thing that ever happened to the Monkees was to have Hollywood write and perform the songs for them, while they lipsynced, clowned around and looked good. The best songwriters in the business wrote their songs. How lucky could they get?
The Monkees were not ripped off in any way. The value they received was becoming famous, and that has enormous value they can use to make money off of anytime they please (and they have done so).
One critical value they once provided was to discover and popularize new styles. They don't do that anymore at all. Last time they did in a big way was probably grunge in the early 90s. Since then any new movements have had to go around the major labels. They've retreated to endless retreads of copycat pop.
It's not that they were benevolent, but 25 years ago they held the keys to the castle and disrupting production, distribution, and marketing was nearly impossible. Today with the rise of CDbaby and other agencies handling piecemeal work via the Internet (and bandwidth opening up to serve media), they're dead.
But until recently, their services were quite necessary.