Why Are Multiple Layers Better Than One?
The way your clothing keeps you warm is by trapping a layer of warm air between you and it, which then acts as insulation.
When you have more than one layer on this creates multiple air layers which trap the warm air, this provides efficient insulation as when the air escapes from your innermost air layer it then gets trapped at the next one instead of escaping straight out.
If you were to just wear one layer (even if this was your biggest, thickest jacket) that one air gap can’t hold as much warm air, and doesn’t provide as much of a barrier to stop the warm air escaping.
Having multiple layers also means you have more control over your temperature. If you’ve only got one layer to wear, you’re more likely to find that you’re either too hot or too cold.
While if you have multiple layers, you can add or remove layers as needed to provide more or less warmth, wind proofing or waterproofing as needed.
For example you may be planning a half days walk up a hill on a cool day, you’re wearing your wicking t-shirt and thermal jacket, your bag is packed and you’ve got your water bottle at the ready. You start your walk, but after a few hours the weather turns and suddenly it’s raining; if you’re following the rules of layering, you’ll have a waterproof jacket you can throw on to help keep dry, which you can take off again once it stops raining.