A warm bath before bed can help you get to sleep. It is not the rise in body temperature that matters most, but the cooling down that follows. That can make the body seem as if day is ending and night-time is beginning.
This may create a stronger urge to sleep. You might notice something similar after sitting in a sauna for a long time. When you step out, you can feel worn out even though you have not done any work.
Part of that feeling comes from the body cooling down. In today’s climate-controlled homes, that natural cooling can be harder to experience as day turns into night. A bath can help imitate it.