A logo is what makes your company recognisable. It's a visual cue that makes your brand into a familiar one, and is often the foundation for all of your marketing materials, from signage to stationary, and of course, your website.
Having a professionally designed logo is important as it becomes a symbol or representation of your brand to your customers. People are more stimulated by images over words and often remember visual elements like logos long after they've forgotten what they've read. What people feel about your brand becomes attached to your logo, which in turn becomes a representation of how they feel about your brand.
A good logo and subsequent branding through the use of colour, shapes, angles and symbols can represent to people things like quality, dependability, power, strength, speed and even durability, qualities they then associate with your company.