Handing out printed forms in class leads to higher response rates than asking students to fill out a questionnaire online. However, it takes up valuable class time.
Having students give feedback online allows them to do it on their own time and provides you with automatically collated, downloadable results. However it does risk obtaining a low response rate, which can make the results less reliable or even biased. To encourage a high response rate some teachers ask students to bring to class a printout of the confirmation page that indicated the student's feedback was submitted - and, as a bonus, some give those students points for class participation.