Organization is key to setting a comic universe. No matter how whimsical or random it is, having a good basic underlying skeleton can allow you to accomplish a lot. It allows you to decide on what is happening next, see what holes exist in your organizations, and get inspired by connections you would not be able to see otherwise. All told, laying it out gives you insight into the big picture of your universe.
A good bible should have three parts. The biggest part is how your universe works. This means that you will need to create a quick history of the world, especially if it is substantially different than our history. You also need to define how science and/or magic works, pointing out major advances and interesting anomalies. You also need to show how common it is; if everyone has a watch that doubles as a supercomputer or magic items are common, this is the place to note it. This section should also detail any major organizations and their rivals, especially if they are important to the story. In short, this should be the biggest section and one that is constantly being added to as you discover new things about your world.
The second section should be character descriptions. This should include not only brief descriptions of the characters, but also artwork showing them in static and dynamic poses. Characters with smaller roles can get briefer descriptions and single pictures, but major characters will need as much space as you feel comfortable giving to them. In general, the more history a character has, the more space that character should get, especially if that history is important to the comic. Yes, there is always the possibility that a character with an incredibly detailed backstory may not get so much a mention, but it's always better to have a lot of information rather than too little. This section should also include any important places or items; that really cool mecha or sword deserves to shine, and this is the section for that.
The third section should be for your plotting. Consider this a continuity dump; any strip you do should get a brief synopsis and tat synopsis put here. This section allows you to plot ahead, and see where your plot has already been. It also helps you keep things straight for when you start getting complex, and that will happen. Better yet, it can also act as inspiration for when writer's block happens; you can comb through looking for loose ends or small threads that can be further developed. It can also allow you to make major changes in the plot, as you have a jumping off point and can easily link it to events that happened in the past.
In short, a bible may seem like an awful lot of work. But it has a number of advantages, not the least of which is that it can serve as inspiration when you need it the most, and it can help you decide if a plot thread is worth it or not. As long as it is properly maintained, you'll find plenty of use for it. It can serve all writers well, especially if you are looking for a great new toy to play with.