I use BeginnerTriathlete.com to keep track of my workouts. It lets me log swim, bike, run, strength and whatever else I can possibly think of. Then it gives me fancy little graphs that track my total distances and times of each sport by day, week, month and even year.
Personally I find this to be an invaluable tool in tracking my training and knowing when to add or decrease workloads. The reason it is important is because in order to prevent injury us endurance athletes need to know how much volume to increase by. Without this knowledge it is kind of like pissing in the wind! Another reason it is important is so you are able to track if your current training is effective. If you go months and you are not seeing improvements then maybe you need to take a look at your training plans. It gives you all of the cumulative informaton in one simple to read layout that allows for quick but accurate analysis.
Now if you want even more detailed information and even online coaching then you can upgrade your membership for a relatively low price. I am probably going to do this in about a month when I start training for an Olympic level race.
The reason I am bringing this all up today is because I added my volume log as a widget to the right ---->>>
Supposedly it will update as I log workouts. It breaks the workouts down by week and each color represents a different sport as the key indicates. You will notice a considerable drop off in my logs last week while I was sick and resting.
If you are exercising and not logging your workouts anywhere I highly recommend this site. It also has a ton of great articles, training plans, and a very active forum. Check it out!