The whole idea of casting Grease is not only to knock your opponents prone but — for certain fire trigger happy wizards — to ignite it on fire (fireball as one option) causing potentially extra damage for the individual(s) in the grease.
What is unclear though is how much extra damage (if any) will the now ignited grease provide?
While the answers might be similar to this question/answers, that question deals with non-magical grease (flask of oil).
The grease spell has the components pork and butter, so I’m wondering if it would be different, or if it would just take the standard 5 points per round.
Grease cannot be ignited.
Spells only do what they say they do, nothing more.
The full text of Grease is:
Slick grease covers the ground in a 10-foot square centered on a point within range and turns it into difficult terrain for the duration.
When the grease appears, each creature standing in its area must succeed on a Dexterity saving throw or fall prone. A creature that enters the area or ends its turn there must also succeed on a Dexterity saving throw or fall prone.
There are no references to the grease being ignitable, so it is not. It’s important to note that not all kinds of grease are flammable in all situations. Pork rinds and butter, the material components for the spell, are not flammable in all situations, for example.
Moreover, Jeremy Crawford has tweeted support for this interpretation.
While it’s a popular houserule to light it on fire, the resulting damage is purely up to the DM.