The purpose of critical thinking is to ensure that people are able to think clearly and rationally about what to do or what to believe. This is very important in any walk of life but especially in the realm of formal education, where it's vital to be able to construct rational arguments in support of one's ideas.
In writing an essay, for example, one is required to put forward a clear argument that shows evidence of reasoning. It's not enough to write down a series of assertions or blind prejudices; one must always be able to back up one's arguments with appropriate evidence.
Critical thinking is essential in this regard, as it allows one to understand the logical connection between ideas. It is only by doing so that one is in a position to present a coherent argument, an essential element in any piece of academic work, whether it's a high school essay or a PhD thesis.
Outside the groves of academe, critical thinking enables us to spot bad arguments, of which there is always an abundant supply, especially on the internet. A cursory glance at the comments section of a news website provides ample evidence of the inability of so many people to construct even vaguely coherent arguments. Critical thinking allows us to cut through such ignorance, prejudice, and bile and gain a better, more nuanced understanding of the relevant issues.
In an era of "fake news" and ever more deranged conspiracy theories, the benefits of critical thinking have never been more obvious.