They don't have the same kind of board power they used to, but they're still very influential. They are often more experienced than founders, and also, being older, more authoritative. And much of their advice is good. Especially the best investors, some of whom are really smart. So founders rightly start to rely on their advice.

The catch is that founders' and investors' interests are not always aligned, and when they aren't you can't trust investors' advice quite as much. But the two types of cases aren't sharply differentiated. Which means to avoid being misled by investors you have to be able to judge precisely how misaligned your interests are in each situation, and discount their advice by exactly that amount. It's a subtle problem.