I have taught English as a Second language to new learners.
The question/answer pair "What is this?" / "This is a ..." is lesson 2 (after introductions "Hello, my name is...", "What's your name?")
t teaches students how to ask for vocabulary words.
It teaches that English uses a change in word order to make a question from a statement.
It is a basis for teaching the word order for adjectives:
"This is a pencil."
"This is a red pencil."
"This is a yellow pencil."
The question/answer pair "What are these?" / "These are..." is the next step.
It reinforces the idea of word order, and the question word "what".
It teaches subject/verb agreement for the most important verb in English, making plurals by adding -s, dropping the article "a" in the plural, and that adjectives are not inflected.
"These are pencils."
"These are red pencils."
"These are yellow pencils."
That's a lot of grammar for one lesson!!
Later on, you can teach more complex rules such as the difference between this (something near) and that (something far) and pronouns such as "it" and "they".