Journaling can be a very freeing exercise, as it allows you to express what is in your mind, letting it all out, while better understanding it.
Journaling allows you to slow your mind down and to better understand the thoughts that you are writing down. This can help deal with events that have occurred throughout your day, and can help reduce stress that could have built up from those events.
Many people have trouble falling asleep and getting a good night's sleep due to too much stress. It prevents them from sleeping by keeping their minds racing through thoughts and through the events that may have caused the stress. Even when they fall asleep, their sleep quality may be very low, they may even be having dreams about the matters that are causing the stress.
Since journaling can help reduce this stress, this could be a great activity to perform before bed, as it can help calm your mind down before getting into bed.