What is the purpose of the Qur’an, and why is the most important thing/book to Muslims?

The Qur’ān is the sacred Book that Muslims believe was revealed to Muhammad (peace and blessings be upon him), the last of the Prophets. The Qur’ān was revealed to him gradually, bit-by-bit over a 23 year period – from the time of his call to be a Prophet in 610CE until shortly before his death in 632CE. Muslims believe that Allāh does not speak directly to humankind, rather He created them, then sent to them Messengers to inform them that Allāh created them and commands them to worship Him. This is in order to distinguish which of humankind will have faith in a Lord they will not see or speak to until after death. Revelation is conveyed to mankind through the Angel Jibreel (Gabriel). Allāh said: “And it is not for any human being that Allah should speak to him except by revelation or from behind a partition or that He sends a messenger to reveal, by His permission, what He wills. Indeed, He is Most High and Wise. And thus We have sent to you (O Muhammad) an Inspiration, and a Mercy of Our Command. You knew not what is the Book, nor what is Faith? But We have made it (this Quran) a light wherewith We guide whosoever of Our slaves We will. And verily, you are indeed guiding mankind to the Straight Path.” (42: 51-53)

Excellent question - The most simple answer is to guide humans. Muslims believe God didn't just create us & leave, but that he actively guides us to be the best we can be. This has come through many different signs & revealed books (like the Torah, & the Bible) when we were at different stages of readiness to accept guidance. The final revelation (the Quran) came to summarise, add to, & finalise the other books - God's final help & guidance to us. Muslims believe it has the principle solutions to all base problems humans will face. This may include how to live a good personal life, a good family life, & develop as a thriving community (or country). Humans (scholars - people who understand) need to take the principles & apply to the problems we see today. E.g. we may think technology such as the internet has changed all this, but it's just a tool. We still deal with the same problems like human emotions - just at a much faster rate because our communication tools (like the internet, Facebook etc.) are much faster. It's a very deep topic that's too complex to fully explain here, but I hope that helped a little! ☺️