Plans are useless, but planning is indispensable. The act of diagramming things up front is useful to get you thinking about the problem space and come up with the outline of a solution. After that, keeping the initial design documents up-to-date is optional, and often might not be necessary.
During the prototyping phase diagrams do help, but they're usually sketches written on paper or whiteboard. You don't want to waste time with tools and strict specifications to design perfect diagrams, mostly because the design will change frequently, and you don't want your tools getting in the way.
Later on, once the design has settled down and maybe once development has started, those initial diagrams are mostly worthless, but you _do_ want neat and professionally done design documents that describe the system. This allows you to share them with coworkers, and quickly onboard other developers to the project. Hopefully by then the design won't change frequently, which would make updating these a chore.