As an example, let’s say we had 3+4x2
Without PEMDAS, we would add 3 and 4, then multiply by 2, getting 14 (the wrong answer).
With PEMDAS, we would multiply 4 and 2, then add 3, getting 11 (the correct answer!).
So if you don’t use PEMDAS, you’ll just get a different answer than you should’ve. Hope that helped!