probably so that if the other one isnt working for whichever reason they can try the other
Some individuals don't have access to certain social media platforms, such as YouTube being blocked in some regions of the world, so it allows for alternatives.
well you can still use other social medias. For example, if youre chinese you can link niconico, reddit and tiktok.
As a recommendation, Discord is a great one to link/
Linking discord is a must. Most speedrunners have it so its the one they will try to use to contact you if you dont answer to your sr.c dms. As a side note. If some servers give you roles depending on what you have accomplished and your sr.c and discord usernames are different they can check your profile and that way its harder to be impersonated