First it is good Netiquette to include the company logo. So, do brand messages with a logo. Given, looks good.
Company Logo Netiquette Signatures
Next, logos are professional images that represent a company or product. And so, graphic image to associate with an organization or product. Then, for branding purposes.
Signatures Company Logo
A good logo in the signature of a message makes it look good. It can make up for spelling errors and grammar mistakes if the company that a person works for looks professional. Good logos look professional.
Company Logo Netiquette
Company logo because it is professional.