Logo / Branding
The importance of branding and particularly a logo cannot be overestimated. They convey information about you to a new customer or client on first impression - and will lead to recognition of your company or product locally, internationally and online.
People don't think in words, they think in ideas and impressions - visual impression being a primary one. So when an existing or past client thinks about you, while they know the product or service and they feel their judgements about you, its your logo and branding that they see.
I believe designing a logo should be a fairly collaborative process between designer and client because you don't stop learning about the client and their business once a brief is agreed.
Reaction can be as enlightening as instruction sometimes and good bespoke design is all about understanding.