You brand identity is one of the best way to promote your product/service A logo designer can make the thing different in the mind of the customers. Agree?
A logo should not make the things different. The logo should make the things exactly like they are, but it should make the customer to notice some things that you can't say explicitly and/or you don't want to say explicitly and/or the customer doesn't notice these things in some other way. The logo is more about "look & feel" than "making the things different in the mind of the customers".
I'd agree with jimairon, in that logos are used to be the 'shiny' representation of a business or organisation that speaks for what they stand for. A good example to show this is when an organisation rebrands itself. If the logo that they had initially was designed professionally, then it should have stood the test of time and therefore the change must be reflecting a changing in the company itself rather than the perception of it by the world around it.