http://en.wikipedia.org/wiki/Mountain_bike_trials
its says: "is a discipline of mountain biking in which the rider attempts to pass through an obstacle course without setting foot to ground."
achieving bigger obstacles are harder, more talented riders can achieve bigger obstacles. everybody wants to be better or even the best rider.
therefore people buy specific bikes to help them do so e.g. high bb and extreme light weight saving. also people will adjust and modify their bikes to be lighter and help the go 'bigger' e.g. low tyre pressures. very forward handlebars and going as far as drilling holes in rims, frames, cranks etc.
all this and hundreds or even thousands of pounds later they can go up a wall 1" higher. seem stupid yes? maybe not to some people but to everyone who is a non trials rider can agree... it appeals to people that trials riders who do this are sad and have a boring life. making trials look boring.