If you bring the journal into the field with you you can record live information- this is ideal for data collection or gesture drawing the draw back is your journal tends to get weathered faster but I prefer to think of it as warn in!  If you bring samples back with you they have the benefit of seeing it out of context with can lead to more focused attention/ interest at a science table in the home. The hard thing is that you are seeing it out of context so you cannot see how your specimen reacts and interacts in its environment and if it was a plant it will deteriorate rather quickly. Going from Memory is a great practice in memory work and teaches them to pay close attention to what they are observing, knowing they won't be able to reference it when they go to write/draw about it. The draw back is that they won't be able to reference it and may record false or fuzzy information.