Good names help to attract first impression and attention. The names display the basic info about your site or business. Moreover, it helps to remember the name of your business easily for other members and customers.
If you don't have a good domain name, no matter how good your website is you'll be missing out on some visitors who see the domain and presume it isn't relevent to their search
good domain names should be easy to remember and can leave a deep impression to vistors even at the first time!
A good domain is flexible, specific, but the best ones solve a problem. For instance, mentor uses niche targeted domains that solve specific problems such as: MLMTrafficFormula.com This domain is specific to MLM and solves a problem most have which is a lack of traffic. You won't be able to do this all the time, but when you can it helps play on the psychology of your potential customer. You know when they go to your site with the domain like MLMTrafficFormula.com they are looking to solve a problem.
Also a good short domain name will build the users trust. If you are selling viagra and your url is fakevi4gr4pharm4cy.ws, instead if onlinepharmacy.com, you will lose a lot of traffic and sales