Would you expect to become a top concert pianist if you never practised your scales? If you didn't practise your serve, would you expect to win Wimbledon? The building blocks underpinning any skill might be difficult and repetitive to learn, but they are essential if you want to be the best. Initially, it can seem that they are holding you back, and stifling your creativity and flair, but if you practise enough, the basics become second-nature, and you get them right without even having to think about it.
I've come across writers who believe that punctuation and grammar aren't important, and that only the creativity matters. I would agree that worrying too much about punctuation and grammar when you're writing a first draft can interrupt your flow. It's especially important that it shouldn't hold children back from following their imagination. However, if you know the rules, you won't be tripped up by uncertainty about them. You also will be less likely to be confused by what you've written when you go back to edit it–and oh, yes–you will need to edit it.
Of course, there are authors who have broken the 'rules', but they have done it deliberately to achieve particular effects, such as Daniel Keyes in Flowers for Algernon, whose narrator is for much of the work educationally 'challenged', or James Joyce, who wanted to replicate the stream-of-consciousness in Ulysses. In these instances, they are not making mistakes; they know what they're doing and are consistent.
Mistakes in grammar, punctuation or spelling put a barrier between your words and your readers. Plus, they make you look amateurish and unreliable. Worse still, sloppy manuscripts make it appear that you don't care about your work, and no writer wants that, do they?