The finger positioning—actually the way the yarn loops around the fingers—controls the overall tension of the yarn. Think of it like the path the thread takes on a sewing machine—all those ups, downs and arounds between the thread spool and the needle to make a nice even stitch on the fabric.
Fortunately the yarn path around the fingers is much more forgiving than the thread path on a sewing machine—get that wrong and…yikes, thread ball and swear words! If you find a finger/yarn position that you're comfortable with while you're learnings knit or crochet, go for it!
When I was learning to knit and crochet the finger positions never made any sense to me either, I just kept getting all tangled up! My solution was to hold the yarn out of the way between my index and middle fingers until I learned to get all the loops in the right place. Not very elegant but it worked for me because I had some tension control as the yarn ran between two fingers. Eventually I naturally settled into the "proper" finger position as my stitch technique improved.