Diagrams are traditionally used during the design process, if they are used at all. I've seen sequence diagrams on some recent projects. Class diagrams and others? Not so much. Often such diagrams represent an idealistic view, lacking detail, and often deliberately disconnected from reality (otherwise, the diagrams would be a real mess.)

Also I have rarely seen diagrams generated from code, the main exception being database ERDs ("reverse engineering.") Usually, those diagrams are also a mess.

Also, I almost forgot to mention: with "Agile", there usually is no design process. We'll just "fix it in the next sprint."