I see them in the battle shop for medium tokens but what do they do? I assume they take stats away, like the strength leech will lower the strength of your pet if you use it on them but what is the point of that?

Each leech takes away 2 points from the stat they go with.

Some people want even stats and may use these to even them out. I once wanted pets with completely zero stats and used leeches to achieve that. It just looked cool.

Other people who are really into battling may be wanting to do a specific "build". There comes a point in training where you need to be careful where you add points because you will eventually run out. So every point counts, and if you are training for strength but you have some extra points in speed, you may want to use a leech so that you can add those points to strength.