This depends upon what kind of copy of Win 10 you have.
– If you upgraded Win 7 or Win 8.1, then no you don't need to buy another license (product key) because you already have one from the upgrade
– If you purchased a computer with Win 10 on it, you already have a license
– If you purchased Win 10 from online or in a box store, you have a license
If you downloaded a stolen pirate copy of Win 10, you need to buy a license (product key) to make it legal
If you legally purchased Windows 10 with the product key included, no.
Product key is for licensing
No it will activate automatically on the internet if you reinstall fresh
You have to have one if you want it to work.