Grammar is necessary to express him/herself correctly. In fact, to communicate in any language we need to form expressions by combining words according to its grammar rules. A word is in its turn just a combination of letters that has a sense according to the dictionary. Therefore, like we cannot obtain a word that has a sense by combining any set of letters, we cannot as well obtain a significant expression by combining words in any manner we want.
It is true that people can usually understand what we mean eventhough we don't respect grammar; but this is not a proof of our ability to express correctly, it is a proof of the intelligence of others, who are able to deduce what we mean from our erroneous expressions (like those of this message, which, I'm sure, won't prevent you of understanding what I would like to express!).