Basically, when using TC, once the coil has reached the temperature you set, it maintains it by the mod adjusting the power. If you vape looking in a mirror, you can see the volts going up and down slightly very quickly. Sometimes you get a rattlesnake sound as the coil is heating/cooling incredibly fast to maintain temperature. Some people don't like this so lower the power to just enough to eliminate it. If it's a good TC chip, you don't tend to get the rattlesnake effect.
Setting the watts/joules (depending on model) to maximum, simply makes the coil reach temperature as fast as possible. There is of course a certain point where it's not worth increasing the power any more that necessary, say if you have a 200W device, depending on your build, you'd probably not ever need to go above 70W as the resistance of nickel/titanium is so small, it doesn't actually take a lot of power to get as hot as it needs. Any mod above 100W IMO is for people like build very low ohms with kanthal.
Some mods, like the Aspire Pegasus and I think one by Eleaf don't even give you the option to set the power in TC mode, they simply set themselves to the maximum and all you can adjust is the temperature.
At the end of the day, if you're in temp mode, all you should be doing is adjusting the temperature and not worrying about the power. Dial it up, let the mod do the thinking! lol