To differentiate you from your competition
Many elements of your brand should differentiate you from your competitors, like your messages, levels of service, the experience, your price point.
A logo's job is to act as a trigger to remind your customers of those differentiations. Therefore your logo needs to be as different as possible to attain that pavlovian reaction especially if you need it to be trademarked.
Here's a few logos we've designed for some wonderful Bristol brands.