For example, let's say I'm trying to gain 15 lbs this year. That's 0.28 lbs per week, or 1.25 lbs per month. My weight might go up and down a couple of pounds throughout the day, depending on the amount of water in my system, when I use the washroom, etc.
So, I weigh myself today, and let's say I had a little extra water in my system. Now, I weigh myself in a month, and I'm slightly dehydrated that day without realizing it. I might have reached my goal and gained 1.25 lbs that month, but the scale is showing I weigh 1 lbs less than when I started.
That's a problem, because I stuck to my goals, my routine, and I get slapped in the face with results that say I'm going in the opposite direction. I might incorrectly make changes to my calorie count, that don't need to be made.
This is why tracking is useful. You step on the scale on a regular basis, and you can see the overall trend over time. In the above scenario, I can see although the scale shows 1 lbs less, it's an off day, and looking at the forecast, I'm on target and did average a gain 1.25 lbs that month.
Now, I don't even have a scale, but that's why I'd like to get one, so I can step on it throughout the day, and see a graph to better summarize what's happening.