If you feel like you should be doing better at the game, and can take criticism and be open minded, and ready to put in the hard work, a coach is the best thing for your game.
You can see by the varied answers of your question that everyone on forums has a different opinion, sometimes only one is right, or both are wrong, or both are kinda right. A coach can give you an exact answer.
I've tried learning from forums and training sites and books, but I only saw drastic improvement after getting a coach. And now I am learning more effectively from training sites and books because of my coach.
You know how Wikipedia can be totally wrong sometimes? Well a fair few of the people on here are just as (un)reliable. You can be getting advice from people who have been stuck at 2NL for their whole poker career, should you really be listening to them?
If you're serious about improving get a coach, and if you listen to them and put in the hard work, you'll wish you had done it sooner