Indeed we now have the technology to have virtual seminars or even entire conferences on-line complete with "coffee breaks", business meetings and dance parties. Why not even hold an established conference, like STOC or SODA in a virtual world? The total cost would be much less than traveling to a real-world meeting and nearly every aspect of the conference experience could be simulated.
One advantage of a real-world conference is not so much what one can do but what a real-world conference prevents you from doing. While away at STOC you can't teach your course, attend committee meetings, hold office hours, meet with students, etc. You are forced by circumstance to reschedule these activities and open up your calendar to see talks and meet with your colleagues. But at a virtual meeting, can you tell your chair you have to miss your class and the faculty meeting while you sit at your computer, your body in your office but your mind in a different place?