Step-by-step explanation:
The ending tells the audience who you are talking about, and also the tense you are talking in. Without the ending, no one would know who you're talking about (if you didn't say the subject before), and everyone could just assume you are always saying the present tense.
Ex. What they think you said- I'm buying a cake for my brother's birthday.
What you actually said- I bought a cake for my brother's birthday
This interpretation could lead to all sorts of problems, I'll leave what to your imagination, and this goes for many other situations.
Last is the obvious: if you do that in class, you fail your test.