The api exported for FoldMs here is the same as that exported for Folds by Control.Foldl. Stock folds like sum, mconcat etc. are defined in terms of FoldM. This was originally done for benchmarking - and indeed it seems there is no particular advantage in using pure folds, even in completely pure environments.
It may be that the module is slightly simpler to use, since it doesn't involve the constant shifting of gears between Fold and FoldM. Many programs should carry over without change, requiring perhaps a move from purely to impurely.