Why scientists assumes that nature have laws? Is it possible to have no laws at all? I mean that how nature behaves may vary with space and time but this doesn't mean it is unpredictable at all. It behave in its own way. If I let an object fall from a height then it could drop/explode/do anything. But something must happen. The fact that predicting this behavior is difficult doesn't mean that nature has no laws at all. So why it is considered a priori that nature has laws?
To answer this question you first have to know what physicists mean when they refer to physical laws.
To a physicist, a physical law is a rule which appears to account for the behavior of a physical system in such a manner as to allow the physicist to apply that rule to predict the outcome of an unperformed experiment or to explain the outcome of a performed experiment.
That rule must 1) furnish the correct answer within its range of applicability and 2) always furnish the correct answer every time it is applied, within its range. The rules of greatest utility in this context are those which can be concisely described in the language of mathematics, which means those physical laws are usually written as equations.
Physicists do not automatically or simplistically assume that nature has laws of this sort. They observe the behavior of physical systems and thereby discover regularities in that behavior that hint at the presence of some sort of underlying principle which might then be mathematically expressible as a "law". Because most working physicists believe that the universe they inhabit is causal, they carry an expectation that the universe contains laws that account for effects in terms of causes, and it is their job to uncover those laws.
A universe without physical laws would be one in which for example a planet could exhibit a stable orbit around a star on one day and then on the next day suddenly become unstable for no cause. If the universe were acausal in this regard, it is unlikely it would contain any humans to take note of it because a planet on which life might evolve would be just as likely to fall into the star as it would to be flung away into deep space at any instant in its existence.
A universe without laws would be one in which counting objects would be not just impossible but meaningless, because objects could pop into existence and wink out of existence at any time. Because of this, the discipline of mathematics could not have a meaningful existence in an acausal universe.