I've only really used long fiber sphagnum and perlite on the bottom of pots to keep the mix from going out the drain holes.
If I fill a pot with only live Sphagnum moss I'll put a layer of perlite on the bottom. The very first Venus Flytrap I had came potted this way and if it worked for the 10+ years I had the plant why not repeat a winning combination. If pushed for a reason why I'll and say "drainage?"
Joseph is the grand master of layering and sandwiching media types. Perhaps if he has the time he'll share his thoughts and experiences in this regards.