Many, but not all, startup founders use advisors to provide guidance in the early days of a company. Advisors can be very useful for two purposes:
If a company's founding team is missing crucial knowledge or connections to get the company to the next level, bringing on an advisor with such knowledge or connections can be a relatively inexpensive way (in terms of both time and dilution) to acquire such knowledge or connections. Advisors are typically paid a small percentage of the equity of the company and provide between a few and a few dozen hours of support each month.
Having prominent individuals on a company's advisory board can often boost the profile of a company. This is especially true if a company is working in a very specialized space, such as biotech, where having a scientific advisory board might help convince investors and customers that the company has been vetted by people who know the space.