If you are looking to create a website, or maybe you already have a website and you are looking to build micro sites or additional sites for other locations you may start to look through lists of domains that you are able to bid on. Some experts say that if you buy domains that already have some authority and weight behind them then you can quickly take advantage of the SEO and help to promote your new site. Often people buy these domains and then build sites on them that will link back to their main site. This is a way of building up backlinks and helping your main site to grow and move up the search engine rankings.
There are risks involved when doing this and you need to really know what you are looking for before you start to bid otherwise you could find yourself spending quite a bit on expired domains that really have no benefit to you.
It is often worth hiring someone that has a lot of experience in this as they will know the places to go to buy the domains and what sort of money you should be spending on them.