In an agile setting, UML can be utilised lightly. The idea is to be clear about the role of modelling in your project and what you anticipate from it.
Class diagrams and sequence diagrams have been shown to be effective in assisting teams in discussing issues. It can communicate things that aren't obvious in the code in a clear and concise manner (or scattered across many source files).
Scott Ambler, for example, has written extensively on UML-based agile modelling. Of course, you won't use it to create an exhaustive model that includes all classes and properties. However, you'd draw the core with a few important classes and only a few features that are relevant to the debate (Ambler says "Just barely enough" modeling).
However, UML requires a level of precision for architectural modelling (deployment diagrams and the like) that is not always feasible in early stages. C4 models have proven themselves as a practical and adaptable option in this area. C4, on the other hand, uses UML to discuss OOP design. There is simply no other option that allows for the easy display of classes and their interactions, and that is widely known.
Conclusion: Don't be fooled into thinking that UML requires a comprehensive up-front design in an agile situation. Don't use it for visual programming, when you slavishly replicate the code's intricacies. However, utilise it as a tool to communicate key ideas and allow everyone to understand the design and contribute meaningfully.
