There are two aspects to brand marketing.
Obviously, your ultimate goal is to get a brand name that is so well known that people look for it directly - people search for 'facebook' or 'amazon' rather than 'social networking' or 'online bookshop'. (And yes, people do search for 'facebook' rather than going directly to facebook.com)
But along the way, you can build up a brand that is at least recognised. There are certain websites that I know from experience are quite good, and so when one of them comes up in the search results, I'm more likely to look at it because I recognise the branding/sitename. Yes, you need to get your site high up in the SERPs to benefit from this, but it can be very useful in improving the click-through rate from search results pages.