Low costs. Due to drastically lower overhead, collecting data does not have to cost you thousands of dollars.
Automation and real-time access. Respondents input their own data, and it is automatically stored electronically. Analysis thus becomes easier and can be streamlined, and is available immediately.
Less time. Rapid deployment and return times are possible with online surveys that cannot be attained by traditional methods. If you have bad contact information for some respondents, you'll know it almost right after you've sent out your surveys.
Convenience for respondents. They can answer questions on their schedule, at their pace, and can even start a survey at one time, stop, and complete it later.
Design flexibility. Surveys can be programmed even if they are very complex. Intricate skip patterns and logic can be employed seamlessly. You can also require that respondents provide only one response to single-choice questions, which cuts down on error.
No interviewer. Respondents may be more willing to share personal information because they're not disclosing it directly to another person. Interviewers can also influence responses in some cases.