Scupper plugs?

I just bought my first SOT kayak. I see that there a plugs for the scupper holes.

What is the purpose of using these plugs? Is it to prevent water from coming up through the bottom of the boat or is there a different/additional purpose?

Got tired of sitting in pond scum…
…and filled mine in with Marine Tex.

I did install a couple of male brass hose ends with caps so I could open them up if I wanted.

Worked great. GH

…for the info, everyone!