Props make a play more interesting in many ways. Without props, a play seems less life-like. Very rarely does a person walk around and not interact with anything around him or her. People touch things, hold things, gesture to things. Without props, actors would have nothing to interact with, nothing to make them seem more genuine.
Props allow the play's audience to better visualize the events of the play. Sure, an actor can hold up his hand and pretend there is a vase of flowers there as he makes his dramatic monologue, but without the physical object of said vase of flowers, it is difficult for the audience to be equally focused on imagining the prop. Additionally, props give the audience members more to look at as they watch the play, which subsequently makes the play-going experience more interesting. Even the most intent play-goer's eyes are capable of taking in the surroundings of the actors and props provide the extra fluff needed to maintain audience attention.