1 Alvissa: So, Kyros, why do you seek to obtain more power? You're not turning into an evil wizard, are you?
1 Kyros: Turning into? No.
2 Alvissa: You could build a tower and retire to a life of magical research.
3 Kyros: Where's the fun in that? It's all summoning demons and trying not to get your soul devoured.
4 Alvissa: Research wizards don't have to summon demons.
4 Kyros: Try telling that to the guild.

It seems to be a thing. Every map of a wizard's tower I've ever seen has had a summoning circle in it somewhere.

If they're not using it for summoning demons, what are they using it for? Summoning pizza deliveries?

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Reader Felix writes:

The annotation does presume that such circles/pentacles are only usable for summoning spells. Depending on the tradition of magic (for "real life" usage) or rules of magic in the setting (more fictional), a "summoning circle" might act as the thaumaturgical equivalent of a fume cupboard (limiting the effects and/or side-effects of a spell to a defined area); or an emergency shelter (with protective wards along the perimeter to prevent anything from harming the wizard cowering within). It may also be the altar where a sorcerer pumps in sacrifices, magical energy, and reagents to effect a more powerful spell elsewhere; or it may simply be a ritual aid to get them the right mage-y state of mind.

That said, summoning demons, spirits, and nameless dead gods from beyond the dawn of time seems to be half the fun of wizardry...

...and summoning pizza is far more practical than the other things (presumably drunk and/or bored) wizards get up to - owlbears anyone?