I’m not to familiar with the plantaris muscle, other than it is considered “vestigal.”
of interdigitations strengthen the argument that the plantaris muscle supplement
the activity of the lateral head of the gastrocnemius muscle whereas the
patellar extension suggests an involvement with patellofemoral dynamics and
may play a role in the various presentations of patellofemoral pain syndrome.”
The gastrocnemius in the above is one of the big muscles in the calf and plantaris is thought to help this muscle rather act as a strong muscle its self. But for all the talk of it's being useless, a more honest statement would be that we don't know it's function in humans.
Ignorance of function certainly is not proof of no function, as with “junk” DNA, appendix, etc.