My artist journal is a place where I try things out. Sometimes it is a concept. In these pages, however, is was experimenting with materials.
I had some new metallic watercolors, see the painting at the bottom where I experimented with them. Also in these pages I play with collaging different type of papers, layering paint in different ways, trying using a little perforating roller to make tiny white dots (second to last below), trying a new pattern by using a credit card in black ink, and considering different color combinations.
The advantage of doing these things in a journal cannot be overstated. You can look back on them and learn from them, remember them and use them again, or avoid doing those things in the future. Collecting your trial and error in a way you enjoy revisiting it is wonderful for growing any art practice. I encourage you to give it a try.