Puck's breaking of the fourth wall is important as it highlights the unreal nature of the action that has just unfolded. The entire play takes place in an enchanted, dream-like world, where nothing is quite what it seems. And right throughout the play Shakespeare successfully maintains this dream-like facade, presenting his audience with an alternative reality.

That is, until Puck turns to the audience as he makes his final speech. By breaking the fourth wall, he hopes to make amends for any offense that may have been caused. The entire play was just a dream, with the audience in the state of collective slumber. By turning to that audience, Puck is waking them up, and in doing so "mending" any damage done by the chaos and disorder that they have just witnessed.

There are two answers to this question. One has to do with how a play is written and the other with how a play is performed.

Puck’s final speech is an epilogue. In classical theatre epilogues are speeches that wrap up the action and are often addressed to the audience. So if you are looking at how the play is written, Puck’s speech would be an epilogue addressed to the audience.

The performance technique that you are referring to is called “breaking the fourth wall.” In most theatre the actors act as though the audience isn’t there, as though there is a wall between them and the audience. They don’t make eye contact with or speak to the audience. But sometimes, often when an actor is alone on stage, they break the fourth wall and talk directly to the audience. That is how Puck’s epilogue is performed: spoken directly to the audience.