If you are adding a multiple choice question where the answer choices you provide are not exhaustive, then having an Other choice field is really useful. It means you can list the most common options, but ensures that the respondent has the option to provide their own answer if none of those given are correct.
For questions where an answer choice could require a further explanation, or where you'd like the respondent to explain their answer choice you may want to give the respondents the freedom to add their own comments with the Comment field.
When combined with Text Analysis in the results, both the Comment and Other fields can provide very powerful insights that you could have missed otherwise.
To add an Other choice:
To add a Comment field: