Everything depends situation to situation, but the idea for fair is to stop their momentum and force or steal their second jump while leaving them in hitstun. Ideally they dont get hit by the last hit unless its going to stage spike them, which can be timed if theyre recovering from below you. Weak nair could potentially be better as far as the hit goes, but has less range, less hitstun, and doesnt have as long a hitbox. Fair often wont gimp on its own, but it leaves the opponent in a very vulnerable position. It's especially useful right near the ledge. I use it against olimar, spacies, wario, lucario, and others.
Another very good tool is the second hitbox of pikas uair. Time right itll knock the opponent down and away from pika offstage. Its really useful if pikachu is hanging on the ledge and needs to keep an opponent offstage and theyre either in back of or back and below pika. Its good on characters like kirby, earthbound kids, spacies using their up-bs from that spot, and others.