In addition to the obvious "green" advantage of saving paper, colleges prefer getting the materials online rather than having staff members opening and sorting thousands of envelopes. Many colleges have offered to waive or reduce the application fee for those who apply online, which could be a significant savings if you apply to a large number of schools. And a significant number of schools this year are requiring that applications be submitted online.
Be aware that it is not advisable to "mix and match." The Common Application warns: "We strongly encourage you to consider applying online. It's quick, easy, and secure. However, if you prefer to print a hard copy, complete it by hand, then mail it in, you can find copies of all our forms in PDF format...Note: Please do not mix-and-match your application, supplement, and payment between online and paper submission. Either submit the application, supplement and payment (or fee waiver) online, or submit them all by mail."
If the school to which you are applying does not take the Common Application, you should go directly to the school’s website and follow their individual instructions for the application process.