When you first start learning Japanese, you most likely start out with romaji. Romaji is using an alphabet you already know to write and read Japanese.
And when you get started out, romaji is great! “Pff, I don’t need to learn kanji,” you might think, “This romaji stuff is easy!”
Slow down there, sparky! Romaji might be easy for a while, but it’s just a matter of time before romaji becomes a huge headache for you.