The way I hear it Peacock brings a very different solo voice to the music than Jarrett does, less lyrical and rhapsodic and more cerebral and mathematical if you will,* offering a contrasting style to the mix. In tandem with De Johnette he's able to bring differing degrees of swing and groove, and in fact I think Peacock is central to the uniqueness of the trio, adding a distinctness to it.
*I don't know if I'm explaining what I mean well, but I've been playing bass more than anything the last few years and that is how Peacock's soloing seems to be to me as opposed to an outright swinging or very melodic soloing.