Wizards and Witches both realise that the accepted good practice, on raising spirits, Demons, anthropomorphic personification or other supernatural entity, is to banish it once the need for its presence is over.
This can be done out of prudence - you don't want a loose demon hanging around unconstrained by a magic circle, if you can possibly help it. Or else out of a sense of tidiness - like dotting the "i"'s and crossing the "t"'s. You started the ritual, you have to end it.
Most often, though, a banishing ritual is done out of politeness and courtesy towards the dread and infernal creature you have just dragged away from what it happened to be doing on its own chthonic plane of existence, much to its inconvenience, in order to answer your pathetic and parochial little questions.
Magrat Garlick, in Wyrd Sisters, tends towards the bookish and the formal, following a conjuration that verges dangerously on being wizardly. She intones the Umm...begone foul fiend... stuff that could have come straight out of a wizard's grimoire. (Elsewhere in the opus, Death objects to being subject to all that "foul fiend" stuff at the end of the Rite of AshkEnte, as he doesn't think it's really necessary, leading the wizards to improvise, and humbly add a hope that they haven't inconvenienced him.)
Rincewind, on discovering the Red Army and how he can manipulate their actions, with a wild and not-altogether-maturely adult glee, he has them practicing the upward thrust of the middle finger that we are assured is the surefire way of banishing a no-longer-wanted entity. (Interesting Times)