Consider this dialogue:
Alice gets a phone call from her friend Bob.
Bob: My car is broken. Can you tell me how to fix it?
Alice: I guess. What's wrong with it?
Bob: I don't know, it doesn't start.
Alice: Well, you could try jumping it.
Bob: And that will definitely fix it?
Alice: It will if the only problem is a dead battery, but there might be something else wrong with it.
You can't give good advice on how to fix a car, if you don't know what the problem with the car is. You can give advice for the most common problems, and you can give advice about how to troubleshoot it, but whether that advice helps depends on whether the problem with the car is something your advice covers. Unless you can give out a detailed technical manual of every part of the car, some problems won't be covered.
Humans are more complicated than cars, and we haven't figured out the fully detailed technical manuals for them yet.
Therefore, all advice is highly situational. Advice that can help one person could hurt someone else.
Which is not to say that advice is bad or worthless. Advice from someone who knows the situation can be very helpful. And even without context, there are many issues that many people share, and it can be helpful to point out common problems. But, any advice that's given without context (for example, self-help books, or random blog posts) should be carefully considered before being put into action.
That especially applies when it's advice that you agree with, advice that makes you say, "Yeah, that's a great idea!". If you feel like that about some piece of advice, then you probably already believe it, and that belief probably already influences your actions. It won't be in the exact same way that the advice says (otherwise your reaction would have been, "Well, duh.") but it's probably more or less in the same direction. Which implies that the advice is less likely to be the bottleneck to improve whatever it is you're trying to improve.