I really don?t know why they do it. Horses do the same and owls move their heads from side to side to get a good look at things.
So do people, if you watch them carefully! I suspect it is for ranging and identification; getting a slightly different perspective on the ?target? to confirm what it is, especially if there are branches etc in the line of sight.
It is completely instinctive, rather than a deliberate action.
You are going about finding all about deer in exactly the right way; watch, listen, learn.
The only caveat is always to treat the opinions of others with caution and confirm them by your own observation.
I?m sure you won?t need me to remind you that there are a fearful lot of bullshitters in the deer world!