A Journal Just for YouA popular suggestion is to write down what you need to do the next day before you go to bed. That way you sleep better. Writing things down gets them out of your head. You really need that in genealogy. You can NOT carry it all in your head. But sometimes all those charts and forms (or a blank page) are intimidating. Trying to create something formal can sometimes be a hindrance. Enter the genealogy journal.
I think one of the most important aspects of the journal is that you create it just for you, for no one else to see. That should give you a clue about using it appropriately. Another genealogist can benefit from your notes, plans, logs, and reports. Don't put them in your journal.The benefit of a genealogy journal is simply in catching your random ideas and thoughts, not its use farther down the road. Don't confuse it with other types of journals/diaries.
Just WriteYou should write about whatever genealogy topic you choose. You don't need any reference material in front of you when you write. If you have a question that depends on information about an ancestor, don't look it up. Write down what you need to look-up and the implications. Even if it turns out the ancestor is the wrong age (or whatever information you needed), the general idea will probably apply somewhere else in your research. If you have the time and inclination to pull out your notes, you should probably be creating a formal document. You don't have to sit down to solve a particular problem or task although you could. If you feel blocked trying to write a plan or organize a report, you might want to start with a journal entry. Just make sure relevant ideas make it to the plan or report.
I know I hate reading my own unedited work. If you feel the same, here's the good news. You only need to reread what you wrote one time. You need to transfer relevant ideas to an actionable document. Make a to-do list, add it to a plan or log, create a note or report, whatever is appropriate.
Improving Your Skill SetRealizing you need to review existing research may be the most useful feature of your journal. You may realize you need to correlate and analyze data/sources you already have. Don't know what correlating and analyzing are? Those are the formal terms for a very important set of skills. If you have the urge to make a chart to compare information, you want to correlate the data. If you begin having questions about a source you used, you are feeling the need to analyze it. Depending on the level of detail you recall, you might do some analyzing while writing in your journal. In that case, you need to transfer that information to a memo/report or, at least, a note in your file. It's not usually possible to correlate data without having your notes in front of you (which I've already said you don't do).
Your journal can help you formulate genealogical questions you would never have thought of by filling out a family group sheet. You may start to ask questions that require you to develop new skills (like correlating and analyzing) or you may ask questions you simply need to research the answer ("how old did you have to be to witness that kind of document?").
Don't Abuse ItIf you decide to start keeping a genealogy journal, make sure you don't abuse it. Genealogy is a long term project. You need a great organizing system and a long-term organizing system. Your genealogy journal is an informal place for brainstorming or just to get ideas out of your head. If you need to act on anything you've written, you need to transfer that action to the appropriate place. Your journal should NOT be the first place you go when it's time to research.
Is It for You?Finally, you don't NEED to keep a genealogy journal. Just like you don't NEED to do mind mapping. It works for some people and is extremely helpful to them. You do NEED to keep a research log and take good notes. A journal is a suggestion that might be helpful if you have too many ideas in your head or just need to "talk it out" but can't. When you are a less experienced genealogist, it might help you start writing when you don't know where to start. If you just like keeping a diary, this might work for you. Whatever the reason, you may find a genealogy journal helpful.
I've suggested a paper journal but you can keep it in any portable format you like. That might be electronic and with a smartphone, audio is also an option. You can also create a workable solution for any combination of the three with an app like Evernote. Find the format that works for you and don't abuse it. Your genealogy journal can help you think outside the family group sheet.