Why does Jacques narrate the seven ages in As You Like It?

Answer 1

Jaques does not really "narrate" the speech; nobody comes up to him and says, "Hey Jaques, can you do that seven ages speech?" It flows from a discussion in the forest and from Jaques's own melancholy nature. Orlando has arrived at the forest camp and has found to his surprise that Duke Senior is happy to offer him food and Orlando does not have to rob him of it. Orlando goes off to find his aged servant Adam and bring him so he can eat.

The Duke says "Thou seest we are not all alone unhappy: this wide and universal theatre presents more woeful pageants than the scene wherein we play in." The Duke says to Jaques "See? There are some worse off than we are". But Jaques ignores this and instead riffs on the metaphor of "the theatre of life" which the Duke suggests, repeating and amplifying the idea that we are only playing parts in life like actors.

There is a practical reason why Shakespeare had Jaques go on at such length. Orlando has just left to find and bring back Adam. It would have looked silly to have him return immediately after leaving. The speech is therefore a time-filler, so that we know that it took Orlando some time to find Adam and return to the camp. Some of Shakespeare's most eloquent speeches have just such a prosaic purpose.