Making questions required
By default, survey questions are optional: they can be answered or skipped by users. However, admins can opt to make a specific question (or questions,) or fields required. What this means is that the user will be prompted to enter data into, or make selections for, any questions that were skipped. If a user clicks an end-of-page submit button or tries to advance to another step, they will be prompted to answer any required questions before they are allowed to advance further or complete the survey.
Because most questions' responses are captured by the system immediately before the user clicks a submit button, there is no preventing a user from leaving a survey partially completed (closing the browser or abandoning the survey at any point) even if a question or several questions are required. In the event that the participant does not opt to return to required questions when prompted, their input up until that point will still be captured.
For this reason, the most effective way to ensure that a user answers required questions before working through the entire survey is to use Steps. Dividing the survey into steps and dispersing required questions into the various steps will prevent the user from advancing through the survey if questions on a given step have been left unanswered.
Pros to enforcing required questions
Enforcing that required questions be completed before the user can advance in the survey can be useful if certain key data is necessary for the survey's purposes. Perhaps you need the user to specify which organization they are with; making a question for this information required and placing it on its own step at the beginning of the survey will ensure that this information is captured before the user continues entering more data.
Cons to enforcing required questions
Requiring questions can be seen as a barrier to entry, and increases the risk of survey abandonment. A user who is not comfortable answering a given question and being compelled to answer it may opt to walk away from the survey. It's worth considering whether it's more valuable to get some data than no data.
Q: There are 3 options under any question's Advanced Options: Default, Not Required, and Required. What happens if I leave this set to the "Default" option?
A: All questions are not required by Default. Therefore, the "Default" options means the question will not be required.
Q: Can I choose to only require specific fields in a Contact Information question?
A: Yes: be sure to select "show" next to all fields you'd like to collect, and then you can check the box in the "Require" Column to require any/all fields.