Mastering isn`t just a vinyl only process.
It`s an art unto itself.
It`s adding the final polish and sheen to a finished mix, giving it extra prescence and compressing it in a way, that`ll make it sound good on anything.

It`s very hard to master your own stuff effectively, as you are so involved with the track, and will get precious over parts.
An outside ear is good to make the best of what is there.

you trust your ears are telling you it sounds right then mastering is all academic?
Couldn`t be further from the truth.

As I`ve moved into learning effective mastering I`ve found it is a very deep subject, and it takes a while to master it, as it`s mainly down to the experience of your ears.
A good master will turn a good track, that isn`t quite pro, or releasable. Into a releasable and pro sounding track.