You don’t need to - but:
UML notation is well-supported by various (free and commercial) tools, has a clear semantic and is pretty much standardized - therefore we (Peter + Gernot) like it.
On the other hand, UML does not guarantee any positive effects for your architecture or your team. Other notations might work as good (or bad) as UML.
You need to ensure that all stakeholders have a mutual understanding of your notation. That can become really difficult, if you draw only boxes and lines - as those have no semantic at all (at least without further explanation)
People tend to interpret such symbols in a way they like - most likely different than the original author intented…
See question D-4.