Riff recognizes the obvious tensions between his gang of The Jets and the upstart, Puerto Rican gang of The Sharks. Riff knows that Tony, the person with whom The Jets, started is going to be needed as a show of strength and force. Tony is ambivalent at the request, as his life is being defined in a trajectory where the Jets is part of his past. Riff lures Tony with the idea that there might be "something" waiting for him at the dance. It is evident that Riff is intimating at something to appeal to solidarity with The Jets, his former gang. Yet, Tony is on another plane, using the hint to launch into his song of "Something's Coming." Riff's request of Tony is motivated by his need of the present. He understands that there is an existential threat to The Jets posed by The Sharks. His need for Tony at his side is a reminder of the past glory of the gang and also a reminder for all as to what the gang stood for even before the emergence of this new and upstart gang. For Tony, the request to be there is more of an exploration of a conditional future, something that Tony understands has little in way of definition, but is firmly embedded in what is not in the present. While both Tony and Riff have different intent behind their attendance at the dance, their worlds converge into this moment, and Tony honors Riff's request to be at the dance.
West Side Story