As expounded by John Austin, Law is the command of the sovereign backed by sanctions. It is to be noted here that command is the wish of the sovereign while sovereign is a politically superior body subjected to no body.
We need to obey the law to maintain equilibrium in society. Peace and order is the main priority for the smooth functioning in a society. In every walk of life, we need some specific set of laws to govern the conduct of the said field and to make sure that everyone follows that respective law, we need to have a coercive factor backing up that law. For instance, Mr.A breaks a traffic signal and he is charged with a fine of $10, what if there was no penalty for breaking the signal? Everyone would be breaking that signal and the traffic signal will be a mere set of colors at the top of the pole and hence there would be a higher amount of accidents and casualties, God forbid.
Attached video is just the right explanation on the obedience of law.