Hi Jasmine,
Thank you for raising this in the community forum.
Custom fields offer a placeholder value which could serve as a suggestive default to a user but unless you make the field required it can still be left blank.
One nice way to go about this would be to make an Automatic Rule which would fire when an Incident is created and one of its custom fields does not have a value. No Workflows would be needed for this if the default text is always the same as you can assign some new value from the Rule Builder.
Here is a screenshot demonstrating an example with an Incident's description:
------------------------------
Ryan Gordon
Security Software Engineer
IBM
------------------------------