"Mr. Hemmingway, do you write books?" "I do."
"Did Mr. Hemmingway write this book?" "He did."
Just as the pronoun "he" or "I" stands in place of the noun "Mr. Hemmingway", so the verb "do" or "did" stands in place of the very "write" or "wrote". This is complicated by the way in which English uses "do" or "did" as an auxiliary verb in questions: maybe it could be argued that "do" or "did" in the answer is merely the same verb, "do" or "did", that is the main verb in the question. That "do", "does", or "did" would stand in place of another verb when it is expected to be understood what the other verb is, is very far from the only way in which the verb "do" is used in English. However, this inspires a question: might there be, in some languages, some verbs whose only function is to bear the same relation to verbs in general that pronouns bear to nouns?
Act
do, as in What he did was read the angle wrong; do-support do, as in He didn't have any choice, did he?; be with predicate adjectives, as in He isn't certified yet, but she has been for a year, and so on. There's a lot of use for them.