I am new to this stuff, and it is my first time seeing teeth on the backside of a curved saw blade.
This permits (if you are adequately determined) starting a cut in the middle of a board, rather than starting from the edge as usual.
Most commonly seen actually used when starting a hole in the floor, for instance. Once you cut the full height of the blade you'd switch to cutting with the normal cutting edge, which will be much faster and less effort.
Of course using a powered hand-held circular saw is also faster and less effort, so fewer people are familiar with this "feature" on handsaws these days.