A lot of yall don't understand the music business and how music is created. Even vocal production and arrangements can be very intricate and requires a keen ear. So even if you didn't write the song and didn't have anything to do with the beat you can produce by telling the artist how to sing the lyrics and telling the engineer how to layer the vocals and which takes to scrap or keep etc.. that's all part of being a producer and it's not easy to do on a high level.
Think about it this way. If what diddy did was so easy there would be a lot more diddys. But there ain't. Everyone doesn't know how to make a hit song or hit album.
A producer in the traditional sense is more like a movie director. Like Martin Scorsese may not be the one writing the script, designing the sets, acting the lines, hitting the boards in the editing room but he's overseeing and dictating every step of the process and getting the team to carry out his vision.
I work in advertising as a copywriter, and my job is basically coming up with some shyt that satisfies my creative director's vision. The creative director is basically my producer in this case. They'll tell me, "Hey come up with a line/campaign" or whatever, I'll give them my ideas and work and they'll give feedback to make it better, revise it or whatever, and then give me directions on how to proceed. Like yeah I'm the one doing the actual work and concepting, but I'm following somebody else's lead and they play a big part in the final product. So essentially, I'm the beatmaker/session musician/whatever and my CD is the producer