I've just added an online ticket booking system for a client we manage the website for the Stroke Comedy Club in Lichfield. Nice and simple for the client to use and administer, and as the events are very popular it allows advance booking even further in advance. Which is clearly good for our client as he has the comfort of knowing which shows to promote more, and can sell more than just "next show" tickets - all future show tickets.
For customers there's less disappointment now; as shows are always sell outs and late comers end up missing out - now they can book well in advance from their computers, mobiles, iPads, etc.
Online event ticket booking systems can handle sports, gyms, shows, gigs, running events, etc. They all have the ability to build up a database of customers - for emailing campaigns. And some have the option to add in extras to the buying process - E.G. sell other goods at point of sale to increase average customer revenue - t-shirts, posters, whatever customers might be interested in.