They do it because, for each work, they bought one copy, which they scan and no longer need the physical version of, and would be in copyright violation if they keep more copies than they bought.

There was a court case where they said that if they copied the books it's not fair use because they didn't own it but if they bought physical copies and then destroyed them then somehow it was fair use because it fell into the niche of personal backups. I forget the details but basically they buy one time prints and destroy them immediately.

By destroying them they don't copy only convert them into another format.