Heating a single plate of leftovers in a full-sized oven means waiting for the whole appliance to reach temperature, using considerably more energy and time than the small amount of food actually requires to be properly warmed through.
Matching the appliance to the portion
A microwave heats a single portion in a matter of minutes, without needing to warm up first, which makes it the obvious choice for a quick lunch or a small amount of leftovers from the night before that only one person is eating.
For food that benefits from a bit of texture, such as rice or vegetables that would turn soft in a microwave, a small pan on the hob does the job just as quickly as the oven would, while heating only the pan itself rather than a whole cavity of air.
Covering the pan with a lid while reheating helps the food warm through more evenly and more quickly, in much the same way as with cooking a fresh meal, and stops moisture from escaping unnecessarily into the kitchen.
The oven still makes sense for larger quantities or dishes that genuinely need its size and even heat, so this is not about avoiding it altogether, only about choosing it less often for something as simple as reheating one plate of food.
Once the microwave or a small pan becomes the automatic choice for single portions, the oven gets reserved for the occasions where it genuinely adds something, and warming up a small meal stops feeling like a bigger task than it needs to be.
A small toaster oven or grill can also fill this gap nicely for things like a single slice of pizza or a small portion that benefits from a bit of crisping, without needing the whole main oven switched on for such a modest amount of food.
