Yes, of course you're right that the reference to "plays" plural suggests they are advertising the kind of performance they do. Much clearer idea, and it cleared it up for me.
The song precedes the specific "play" they will do "tonight" - about Pippin, introduced in "scene one" so to speak, after the prologue, so of course it refers to what the players do as a profession; what they have to offer, their theatrical 'wares'.
So, isn't it interesting? Fosse is borrowing directly from the Prince device of "Wilkommen" and "Life Is What You Do" and "Tradition" (so it's actually a Robbins device isn't it?) and of course "Comedy Tonight" itself - introducing the style as well as a "company" and milieu, all at once, before beginning the - for want of a better word - concrete story of that evening (as opposed to the abstract framing device). It's one of the tell-tale indicators of what became known as the 'concept' musical.
Speaking of borrowing, Pippin has exactly the same theme as "The Fantasticks" (which in its own way has the framing device song just before the story of the night is introduced): that beneath the lights and colour can lie a terrible darkness. Fosse was bold to go into the dark so flashily.