I've found that a lot of songs recorded pre-1970 will have differing audio levels for different instruments due to their positions relative to the mic, such as with Sinatra's earlier recordings, giving them 'oldie' vibes and making instruments distinct from each other, almost like you're there in some cases.
In terms of doing it on purpose, Pink Floyd made extensive use of such effects.
On The Run and the intro to Time (which come one after the other on 'Dark Side of the Moon') are probably my favourite examples, but I heavily suggest you check out the rest of their work.