Political theory comprises constructs, or models and principles, that are meant to describe, explain, assess, or project political systems, institutions, and relationships past, present, or future. The use of the word "political" in this context means having to do with power and governance. Political theory is meant to address questions such as these:
Who has power in the society?
Who is powerless in the society?
Who are the governed?
What institutions are powerful?
What institutions are weak?
Is the relationship between the governed and the governor stable?
What are the weaknesses and strengths of various political systems?
How might a political system be improved upon?
How might relationships between institutions make them more effective?
What should the balance of power be between those who govern and those who are governed?
These are important questions to ask and answer in all societies. If they can be addressed, it might be possible to have a better world.
For example, in the United States, the political system is arranged so that there is a balance of power between the federal government and the governments of each state. Since the beginning of the country, this has created a tension, a tension that resulted in the Civil War and that continues to manifest to this day, as states claim their own power in various areas such as education, abortion, and voting, and the federal government tries to assert its own power to maintain some control in these areas. Is this an arrangement that provides a balance of power that results in stability or instability? Is this an arrangement that could cause a second civil war? Perhaps the present system is not working anymore and should be changed somehow. Models can be contructed to ask what if this factor changes or that factor changes. Does a problem lie in the balance of power amongst the federal branches? Or perhaps the problem lies in the enumerated rights of the federal government in the Constitution itself. These are all important questions that political theory tries to answer in the hope of being able to effectuate changes if changes should be effectuated.
To the degree that political theory has any significance at all, it is to the degree that rulers, institutions, and people respect it, trust it, and are willing to listen to it and incorporate its ideas. At the very least, though, it is of use to describe what actually is and how it works.