There are different kinds of boards, each with their own meanings and responsibilities, and companies form different types of boards for different reasons.
Members of a board of directors have formal responsibilities (and liabilities) and, among other things, are part of a company's governance. In addition to advising, this is where shareholders (who are not founders) are represented.
There are also boards of advisors, which most commonly include two main categories (business and scientific - though there are others). Members of advisory boards do not have the same responsibilities and liabilities as directors, and their duties are more, well, advisory. In simple terms:
A business advisory board may assist the founder(s)/CEO with business strategy, fundraising, opening doors (strategic, sales, etc.).
A scientific advisory board would help credentialize the company's scientific efforts.
The type of board one is participating in determines the type of duties, responsibilities and liabilities that the board member may have.