I can mention a lot about this subject with various technical perspectives. But for a beginner, I would like to keep it very short on why it is important to practice on a metronome?
Let me highlight 3 basic reasons for you –
(A) RIGHT HABIT – As humans, we’re not naturally inclined to play music in time. The best live performing artists fail to keep their tempo on metronome (if they have not practiced on metronome). Imagine two students training for 2 years, one with metronome and one without metronome. It is like 1000+ hours of practice and training of mind and fingers either with or without it, becomes part of your habit now.
(B) SENSE OF TIMING – The metronome is constant, it does not forgive, it does not forget. Same way, playing to a metronome makes you consistent. It enables students to develop a subconscious habit and sense of timing.
(C) PLAY DIFFICULT PASSAGE – It help us slow down difficult passages while keeping the pace consistent. This approach enables us to work through challenging passages without making any mistakes and we can gradually increase the speed to more levels as we become more familiar to piece.