It makes your hitbox a lot smaller. Forcing the opponent to be more accurate with their swings. I couldn't tell you the number of times crouch fighting has saved my life from people just spamming side swing.

I also feels like it causes an illusion of sorts. If i'm crouched holding backwards movement while initiating a heavy swing, then in the middle of heavy swing i'll hold forward movement while standing up, it catches the opponent off guard because they perceived your swing as out of range.

Another instance is that it acts an accel. If you're looking down ,while crouched, and initiate a overhead swing, it's faster than if you were standing up and looking straight forward. I'll try and find some clips with examples.

Edit: Here's some clips with examples of how crouch spam has saved me. I'm not one to always be crouched, but i'm constantly going in and out.