Too often, we forget the details of a trip as time goes by. A journal allows you to maintain a permanent record of things like a great restaurant, language quirks, the name of a sculptor or artist, a particularly tasty local food item, etc. If you return to that location, or if a friend asks, you can retrieve and provide this information.
We often look back at these journals, either to remember a detail or simply to relive, in a manner of speaking, the entire trip..