Fortune is useful because it increases the chances of leaf blocks dropping saplings and apples (if they're oak or dark oak leaves). I'm not sure about silk touch. Sorry I don't have any statistics for you.
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Every time you post a question without first consulting the wiki, I die a little inside.
Silk touch on an axe is ONLY beneficial if you're relocating your library. Everything an axe chops quickly drops 1 of itself anyway, and you should use shears on leaves if you really want them.
Fortune.... Most of the stuff an axe chops fastest doesn't drop more than it usually does with Fortune. Except leaves, I think, but the axe will run out really quickly if you go around chopping leaves off everywhere.
It does make you get more of a crop, so it's useful if you're farming and you only have an axe.
So, mostly useless. Enchant it with Efficiency or Unbreaking first, then Fortune if you really want to.