I don't know if everyone else is as much of an avid traveller as I am, but if you are this site is very useful. It helps you find flights, accomodation and possibly travel insurance. It also offers a lot of special deals. Now I haven't been able to look at every different offer on the site or every possible place to travel, so I don't know if it will always be cheaper.
The main purpose of the site is actually it's guidebooks and online guides. Trying to show you how far your money will get you whilst travelling. A few people have claimed that lonelyplanet's guides have got them into trouble, asking for cheaper prices at foreign hotels, but prices change throughout time and their guides generally seem to be in the right ballpark.
The site's guides have many more positive reviews than negative reviews and it has personally served me very well. I especially like reading the online articles about places and even using it for inspiration on where I might like to travel next.
Browsing the site will make you want to travel the world!