A physical event is a bundle of different kinds of interaction, but it’s also a bundle of people at a certain place at a certain date – as soon as you take these things online, that bundle has no meaning.
Conferences are good excuses to meet with people that you would t spend the effort to meet with otherwise. They also collect leads as people come by your booth. They provide a deadline to target for talks, and a chance for people to speak getting a notch on their belt.
Online conferences are just webinars at best. This is terrible if it's boring content (mostly, ROI vendor pitch heavy. But if the content is good, the webinar will be good.
In person conferences have plenty of bad content. Most of it is bad and a waste of time. But, the benefits of meeting with people even out that balance. There's another benefit for most people as well. Going to conferences is a type of vacation, an award even!
To argue against some of this, James Turrell has said that part of the value of Roden Crater’s remoteness is that you have to really care to go there. Getting a plane and a hotel and a ticket, and taking days of time, has some of the same effect for a conference – it gives a selection filter for people who care. There is value in aggregating people around a professional interest graph, and in doing that in a focused way, perhaps even around a particular time. (There are also, of course, exclusionary effects to this.)
Original source: Solving online events, Benedict Evans