And this is a book which we have sent down, bringing blessings,
and confirming the revelations which came before it:
that you may warn the mother of cities and all around her.
Surah al-An’am (The Cattle) 6:92

The above verse says the Quran has three purposes: bless, confirm, and warn. “Bless” and “warn” are simple enough. “Confirm” is the one that confuses people. “Confirm” means “establish the truth or correctness of” (Oxford Dictionaries). The Quran is a witness to the correctness of the previous books.

In truth it confirms what went before it;
for before this he sent down the Taurat
and the Injil as guidance for mankind,
and as the criterion for judging right and wrong.
Surah al-Imran 3:3

The Quran affirms that the Taurat and Injil are true, and calls people to believe their message. Yet, it is sometimes wrongly assumed that the Quran is all a person really needs because it is the last of four heavenly books (Taurat, Zabur, Injil, Quran). But imagine an impatient student in a 4-year university program says, “I don’t really need the first 3 years of study — year 4 is all I need.” It’s obvious to all that the first 3 years of study are crucial in order to be able to grasp the content of year 4. In the same way, the Taurat, Zabur and Injil provide a necessary foundation to adequately understand the Quran.

If you are in doubt about what we have revealed to you,
then ask those who have been reading Al-Kitab before you.
Surah Yunus 10:94

For more information, see a short course called “The Eternal Word.”