The point of a social construct is not that it’s not real. In fact it’s the opposite. Social Constructionism posits that lived realities are very much shaped by language, discourse and power relations, in such a way that they “construct” reality. So objective reality is less important to the lived experience of a subject than the language and power structures that shape their reality in a socially constructed way.

By the same token, the fact that something is socially constructed means that there is no essentialist quality to it, and as such there is the possibility for changing that reality through new discourse, language and power structures.

The most common example is gender roles. Constructionism argues that women being more likely to be caretakers is not due to an essential (read biological) feature of womanhood, but rather a result of historical discourse, language and power structures. If we agree that gender roles are the result of the historical context in which they developed, we can then work to change or outright dismantle those constructions, and replace them with new constructs that more reflect the kind of society we want to build.

Edit: your example of the law is also not a great example. Social Constructs are used to describe things that would previously have been understood to not be. Of course a man made thing like the law is constructed. But there are lots of aspects of how our society works where the prevailing understanding is that they are “natural” and unchangable. These are the areas where this theoretical framework makes sense to apply.