The simile is trying to get us to visualize Santiago's wizened old hands and what they represent. He's an old man, a fisherman who's been out to sea goodness knows how many times over the course of his life. Inevitably, after all that time and all that fishing, his hands will be covered in scars. But not just any old scars. As Hemingway points out, they are
[D]eep-creased scars from handling heavy fish on the cords. But none of these scars were fresh. They were as old as erosions in a fishless desert.
Note the key words here. "Old" emphasizes not just Santiago's age, but how long he's been a fisherman. It also refers to the fact that it's been such a long time since the old man landed a really big fish. And this means that, for Santiago, the sea is now virtually a "fishless desert." Just as the sea has dried up, no longer yielding up its riches, so too has the old man's life. The scars on his hands aren't fresh, because he's no longer fresh; he's not a young man anymore. Santiago's old, gnarled, scar-riddled hands symbolize what he's now become.