Because there's is simply no way you'll ever get sub-20 if you don't know all of them
Ok, that was a joke of course.
There are several reasons why you want to know (or be able to choose between) different algorithms:
- Some are better for one handed
- Some cubers know lots of different algs that start from different angles, which saves time
- Some cubers prefer different kind of moves (for example, some love slice move, some don't). Some like RUF algs, some prefer RUD.
- For FMC you want to use different algs then for speed, probably.
- Some algorithms are easier to memorise, then others. If you don't care about top speed, but still want to be efficient, you can try to find easier algs with a logical structure.
- Some algorithms are harder to execute on some puzzles. For the touch cube for example, I would use algs that have a more logical structure, so I won't get lost in the alg.
- Some people know alternative algs for OLL's to influence the corner permutation, so they can end with U, H and Z perms more often.
- Some people just like to show off with the number of algs they know