Imagine a road in a hypothetical country wherein it is a free-for-all situation regarding traffic rules. You can keep to the left, if you like but none else will object if all of a sudden, you decide to move to the right side of the road. You may take a U-turn anywhere on the road, besides being able to cross red lights at will. Are you already feeling slightly dizzy even imagining such a road? A similar but far graver situation is bound to unfold if it is a free-for-all on the road of language. If every pedestrian and driver moves the way he /she likes, unmindful of what others do, predictably, it is going to create so many avoidable accidents. It is not for nothing that grammar has been likened to the policeman of a language, whose job is to facilitate, regulate and control the flow of thought.
We need a certain uniformity in terms of communicating our thoughts to others so that they are understood in exactly the same manner that we had intended. That is why we all need grammar in every walk of life. If your intention was to convey idea X and if the listener or the reader is getting Y, you can well imagine the misunderstanding, confusion and possibly chaos that can result consequently. Sample these hilarious but extremely telling examples of such linguistic misinterpretation:
Written
Intended
A. Specialist in ladies and other diseases
B. Specialist in ladies' and other diseases
C. Fine for parking
C. Fine for parking
D. Fine, for parking
E. The patient had an alleviation of blood pressure
E. The patient had an alleviation of blood pressure
F. The patient had an elevation of blood pressure
You can see easily that a small mistake here or there can lead to dangerous humor and perfectly avoidable misunderstanding.