I studied Photographic Science at university. A fair bit of that was about process monitoring in the photographic industry and plotting those results against whatever we changed. Basic scientific stuff, vary one thing and monitor what changes. We also looked at long-term movement in processes and tried to correct that drift by varying the chemistry.
Food is an input to a system. Exercise is also an input to a system. I've done this before in a really hardcore way (without quite as much technology feedback) and I know that it is possible to lose weight really effectively by monitoring what you eat, how many extra calories you burn through exercise and then varying one or another a little bit at a time to produce a change in the right direction. It's a pain because it involves a lot of weighing, calculating and writing stuff down. But it does make you ask the question - "Is it worth eating that extra bar of chocolate/slice of bread/bag of crisps?".
So, this is a really obvious thing. If you tweak one of the variables slightly, and weight goes down, that's a good thing. If it's only a minimal thing then it's much easier to maintain. Sustainable, one might say. I'm not starving, I'm feeling better, less tired and less bloated than I previously was.
With the advent of smartphone apps and wrist-worn devices, it's even easier to set up and use these things. I have a device that monitors my all-day heartrate, movement and sleep and that gives me a numerical figure for the amount of calories that I've burned that day. I also use an app that links to the monitor and does all the calculations in relation to exercise, resting calories and just tells me how much I have left. Effectively it's a budgeting app, just for calories rather than money.
I've worked out that I need to burn the amount that the app tells me, minus an additional 200cal to lost a steady 200g a day - effectively a loss of 1400g a week (1.4KG). This is pretty good movement in the correct direction and gives me a good boost when I get on the scales and see another 200g drop off. So, if I want to eat more, I have to exercise more - simple really. The good thing is that this is a sustainable thing and will also allow me to pretty quickly bring the system back into the zero gradient once I get to target weight. From the healthy weight, I'll apply the understanding of process monitoring to keep myself on track.
So, why should you do the same? Well, you don't need to, but it's probably in your interest to do so if you want to maintain or improve your fitness, maintain or reduce your weight. Do you need to spend a load of cash in order to do it? Probably not, but I would recommend choosing a device that does wrist-based HRM and I would also recommend something that has the ability to monitor overnight too.