Depends on how they do it. Normally the idea is not to increase the credibility but it is to increase the marketing of it, you would want more and more people to be interested in the project you do and when you are giving money to people for signature campaigns you are getting a lot in return, people all over bitcointalk will be hearing about your project which will create this "demand" for your project, not a ton but at least people will hear about it and know about it even if they do not get involved and a bit of them will get involved as well.
Of course, the expectation of getting more profit from investments in a particular project is normal. Do you want more confidence and credibility from the project? See who wears the signature, their rank on the bitcointalk's forum and their trust ratings, read why the trust rating was increased. I noticed that the chances of getting a good return on investments in projects, the signatures of which are worn by people with a positive trust, their projects are successful and work further. There may be exceptions, as in any rule.