Mastering. If you know, you know.
If you don't, you'll hopefully learn through trial, error, and experience.
The right ME can make a good mix sound great, and your collection of material sound cohesive. The wrong ME can make a good mix sound like hot trash. Finding one you can work with is worth the cost, in my opinion. Vinyl, download, streaming, or CD. It's all relevant. And a greater understanding/standard on how to approach the different platforms now.
20 years ago, I sent an album to an ME. I had digital overs on a few tracks and had not caught them before sending my files. It was a 3-week write, track, and mix marathon. My ears were toast. When I got the master back, the digital overs were very noticeable on those tracks. As expected, they were nasty and harsh, and we could not listen to those mastered tracks. I asked the ME about it; he said, "those digital artifacts are in your mix," - But he chose to master the tracks anyway. We had to correct the tracks and pseudo-master them ourselves. It still shows.
Doesn't matter that the album wasn't great or that I had too hot of tracks to start with; what mattered to us was the attitude and lack of willingness of the ME. He wasn't willing to be our safeguard and last set of good ears. I never used him again, but I have also never sent files with overs again. 20 years ago, digital wasn't as well defined, loudness wars were in full swing, and we were all still working to balance the sterility.