Pure speculation on my part. In the wild i think the roots help grab anything near it to help it grow vertically. Where there is alot of current, the stem would bend over perhaps enough to let it re-root on the lake or stream bed.
I imagine the plant could also draw in nutrients from the water column from those roots too. Some people dont like their looks so trim them off. I let nature takes it course. Never know where those roots will grab, so its fun to let the plant suprise you