Journaling is a splendid way to help us learn to meditate and reflect on God's Word, as well as make personal application.
Many people do not realize this, but David's psalms (songs) are a record of his meditation and reflection on God, His Word and His works. Throughout Psalm 119, for example, David frequently refers to God's laws, God's ordinances, His precepts, His ways, His testimonies, His commandments and His statutes. He used the gift God gave him for song by writing down his thoughts and insights. Some of the psalms are very personal, reflecting some difficult issues he was facing at the time. Other psalms are majestic and inspirational, reflecting on the wonders of God. All of David's psalms have one thing in common, however; they reflect what the Holy spirit was putting in his heart. Through the divine inspiration of the Holy spirit, David's "journals" become Scripture.
Not everyone can write psalms like David. But most of us can write something. Most of us are capable of writing at least a sentence or two about what God is putting on our hearts.
Writing what God is putting on our hearts—particularly as it relates to His Word—is what journaling is all about.
Recording one's thoughts in a journal is unlike writing in a diary. Diaries provide a detailed record of daily activities and events, what good or bad things happen to us, or what experiences we're having. Journaling is different, however. Journaling is a method of recording one's thoughts, one's feelings, one's insights into things. Whereas diaries are event oriented, journals are thought oriented. Of course, an absolute distinction between a diary and a journal does not need to be mandated. Journal entries often record events, and diary entries, thoughts.
Church history provides us with numerous examples of those who journaled: Augustine, John Wesley, David Livingstone, Amy Carmichael, to name a few. One contemporary model is Jim Elliot.