An established business, for example, should at a minimum consider investing in a website logo for branding purposes. At this stage, you want to communicate brand cohesiveness and start creating a look/feel associated with your business. This doesn't mean you have to break the bank and go fully custom in all areas of your site, but a logo is great start to adding a unique touch to your online presence.
What's the return? Visitors perceive your business as more professional and have a higher sense of trust. A logo can also be used in multiple marketing materials (website, business card, social media profiles). It'll add value to your business as a whole.