Adrien wrote:Why do they need to be repotted every year if it’s the same sphagnum being used?
I typically don't reuse the old sphagnum around the rhizome, but rather in the bottom of the pot. I use fresh moss around the rhizome and it provides many benefits including antibacterial properties and better soil aeration. But even if you reuse the same moss over and over, the problem is that the moss becomes compacted over time, so a repot allows the same moss to regain some of its "airiness" so that it provides more suitable growing conditions for the flytraps.
Adrien wrote:And not repot when the plant out grows it’s pot?
Typically flytraps won't outgrow their pots. They tend to divide so much that they get too crowded to grow optimally. That's another reason that repotting annually is a good idea.