What is, according to Hamlet, the purpose of drama? And what does Hamlet most admire about Horatio?

In Act III, scene 2, Hamlet is giving advice to the players who are about to perform "The Murder of Gonzago". He says the purpose of drama is to imitate life. He has cautions the players not to be too "tame" or to "overdo" their actions because "the purpose of playing, whose end, both at the first and now, was/ and is, to hold, as 'twere, the mirror up to nature...".( III,ii,19-20) In other words, the purpose of acting and, therefore drama, is to come as close to reality as possible. A little later, he praises Horatio for being an "even" or not battered by his emotions. He says, Horatio, "thou art e'en as just a man

As e'er my conversation cop'd withal. ( III,ii,49-50)

Horatio is a little embarrassed by the compliment but Hamlet goes on to say,"Give me that man/ That is not passion's slave, and I will wear him/ In my heart's core, ay, in my heart of heart,/As I do thee."

( III,ii, 67-70) In other words, Horatio is not a slave to his emotions and that is what Hamlet admires in him.