Here's what will happen if you turn comments on - they work with Single & Multiple answer selection questions. You'll let your respondents type in a custom reply without interrupting their workflow. We've known it work great with the "Other" option.

How can I use comments with other type of questions? 

If you are using other question types, such as NPS, rating, smiley scale, you can use survey logic to create an open ended response effect.
(1) Add a text answer question to your survey
(2) Change next step of the answers you want your respondent to comment on as the text answer question you've just added: 

Did this answer your question?