If you wash towels and bed sheets together the towels could get tangled with the bed sheets and then the bed sheets or towels could possibly tear or be destroyed.
Another problem is that it can take longer for the towels to dry than the sheets so you would not likely want to dry the towels and bed sheets together either.
Or washing towels and bed sheets together could cause color changes in the towels or bed sheets due to the dye possibly coming out of the towels and getting into the bed sheets.
It's recommended to separate your bed sheets, towels and clothes and wash them all separately to prevent problems with the laundry.
I always separate my clothes into one basket, towels into another basket and bed sheets get washed on their own every few weeks or so when I have enough bed sheets to wash.