Buying a domain name that contains a keyword/phrase you wish to rank a new site well for does score you some points with Google and co, but it’s not the be all and end all.
If you are lucky enough to buy a domain which as well as being appropriate to the overall content is exactly the search term you want to rank well for (e.g. for the term ‘blue widgets’ you bought bluewidgets.co.uk) you would find this is extremely beneficial for attaining higher search rankings.
However don’t go out buying 10’s of domain names made up of different combinations of a number of your favoured search terms if you have the intention of pointing them all to one site. Doing this won’t help with search rankings at all as search engines will only look at the destination site. The only way these domains may provide some value is if people are typing them directly in to the address bar expecting a site or if you want to stop a competitor from using them.

