@spiritofradio, how much time ya got? ;-) There is a lot that could be said in answer to your question, but I’ll be as brief as possible.
Of all the ingredients that go into the making of music, for me by far the most important is the song (Jagger and Richards are mistaken ;-). For me, the song is to music as the script is to a film. Now, not all music is made with the song itself as the priority or focus. Jazz, for instance. The song structure is used as the starting point for musical improvisation; what the musicians play is arguably more important than the song itself. At least that’s the way I see it. Similarly, I don’t care for avant-garde, abstract film (except those of David Lynch ;-).
I became a music lover a couple of years before albums became the Rock ’n’ Roll format, the 7" 45 RPM single being what it was presented on. It was Pop music: Girl Groups (a LOT of them), R & B sanitized for white people, Elvis Presley, Chuck Berry, Dion (a great singer, by the way), Roy Orbison, The Four Seasons, etc; 2-1/2 minute songs on the radio. In that period, there was an iron-clad way music was made: professional songwriters were employed by music publishers to do writing (much of in the famous Brill Building in NYC), and a number of songs a week were chosen to record; a singer (or singers) was (were) offered the song to record; the chosen singer was sent into the studio to do as the hired producer instructed; a contractor hired studio musicians as requested by the producer (depending on the vision he had for the song).
(As that was going on, Garage and Surf Bands starting making albums that remained largely regional. Once in a while one would break nationally, such as Paul Revere & The Raiders. However, only fanatics cared enough about music to devote that much time, energy, and money to that facet of life).
The Beatles changed all that: A self-contained unit, writing, playing, and singing all the music. Yes, Brian Wilson had been doing the writing for The Beach Boys, they had been doing all the singing, and Brian was producing. But he had The Wrecking Crew playing the instruments on his recordings while the other guys were on the road (including Glen Campbell taking Brian’s place on bass).
Suddenly, ALL groups were now expected to write, play, and sing everything on their albums. How many people are good at all three? In the singles era, only the best songwriters had their material recorded, only the best singers were offered songs, and the studio musicians? All right, we’re finally getting to the point! ;-)
When John Hiatt---all his albums having been commercial failures---was given one last chance to get a hit, he was allowed his choice of any musicians he wanted. Who did he choose? Ry Cooder on guitar, Jim Keltner on drums, and Nick Lowe on bass. Why do you imagine he chose them, out of all the musicians in the world? Because they are not just great on their instruments, but because they are great SONG PLAYERS.
Studio players are expected to play in support of the artist: the songwriter or singer usually. To make the singer sound good, or the featured instrument/musician, or the entire ensemble, not himself. Cooder liked Keltner’s playing so much, he arranged his recordings around JIm’s availability. He is also a favorite of Dylan, John Lennon, George Harrison, Bill Frisell, and Randy Newman. When I heard Keltner’s song parts on Randy’s Good Old Boys album, my concept of what great drumming was about was changed forever. Such artistry, such musicality, such taste!
Yet Keltner in a Modern Drummer interview said he wished he played more like Roger Hawkins. Roger who? ;-) Hawkins was the drummer in the house band at Fame Studios in Muscle Shoals, Alabama, the studio the great Atlantic Records producer Jerry Wexler took his assignments to record. Aretha, Wilson Pickett, Solomon Burke, Dusty Springfield, Bob Dylan, Boz Scaggs, many others. You've heard the term "pocket", right? Hawkins had the deepest pocket I've ever heard.
Playing for the song, or the artist, or the entire ensemble---a very different thing than just being in a band. Different priorities, different skill set. There are many great drummers who could NEVER be a studio musician, Keith Moon perhaps the most obvious. I got offered a recording date by a small-time engineer/producer I know, a single song going onto a movie soundtrack. The basic tracks were recorded with just myself, the singer, and a bass player. After the first take, the producer asked me to "play more" (fills, etc.). I said okay (I was a hired hand, satisfying the customer---the producer---is what it's all about), and played a little busier. It still wasn't enough for him, and he asked me to play more like Keith (apparently a favorite of his ;-). The singer (a son of one of the Lennon Sisters!) asked "This is a 60's-style song. Is that appropriate?" The producer said "Sure, The Who made records in the 60's." I knew what the singer meant, and that the producer didn't. Another take, and he asked for some fills at a particular point in the song. I said "That would walk all over the singer, the melody, the lyrics." To my utter astonishment, he said "Oh, I don't care about that." !!!
I later learned that after I left the studio, the producer laid down his own drum track (he's a multi-instrumentalist), and sent the finished track to the soundtrack producer, who rejected the track. The music producer then sent the mix containing my part, which was excepted. Even famous drummers suffer rejection. Keltner and another drummer were hired by Elton John for the album he produced for Leon Russell. After some recording with both drummers, Leon asked Elton to get rid of Keltner, he wanted more in-the-pocket, "meat 'n' potatoes drumming. What would he have thought of Neil Peart?!