As Vic says, the idea at Lewes is to make sure that people can get into the events they've booked for. Booking in advance gets you a discount as well as assuring you a seat.

Yes, audience changeover can be very irritating. The concerts I went to at Whitby this year weren't like that - everyobdy started queueing at least half an hour before their chosen event and generally clung like grim death to their chairs for the duration, which was generally not more than three hours.

The Gosport and Fareham Easter Festival copes with this by having rolling concerts through the afternoon and evening with 15-minute gaps between acts, which means you can adjust your personal fluid levels without missing anything or annoying the people in the same row, or come in late. It allows the sound men plenty of time to adjust between acts as well. Even with the big venues they use it can still be impossible to get into very popular acts late in a concert.