Punctuation breaks up the flow of written words so that they can be readily understood: for example, each main idea in a passage is contained in a sentence, clearly finishing at a full stop, exclamation mark or question mark; in longer sentences, meaningful subsections are enclosed within punctuation markers, so that these are automatically "related" to the main idea; speech marks enclose what is said or thought or quoted.
Incorrect punctuation, like poor spelling, creates an impression of laziness and / or lack of ability. It is also highly confusing for the reader.