Use it to not get edgehogged on Yoshi's. Sheik's recovery isn't too good and Yoshi's is a good counterpick .Maybe on FD too, though I don't know how useful that would be.
The way Wall Cling works is:
1. There's a set amount of time you can cling before you touch the ground again.
2. You can Wall Jump out of Wall Cling by tapping away from the wall, or you can Double Jump by tapping Up/Jump
3. If you haven't used your Double Jump or Wall Jump(it has a cooldown timer when repeatedly jumping on one wall) it will be consumed when jumping from Wall Cling.
4. If you have used your Double Jump you will be given a new one (only if you jump from the wall, you can't fall off and get another jump.)
5. This means you can Wall Cling, Jump, Wall Jump, Wall Cling, Jump... until you run out of Wall Cling time or your Wall Jumps lose vertical distance.