While they might activate today there is no guarantee they will activate a month from now or whenever you decide to reinstall windows from scratch.
 
 

Lataa Windows 10

 
This is profoundly unfair to PC builders, because large OEMs such as Dell and Lenovo likely pay a tiny fraction of that to put Windows on prebuilt systems though these costs are not made public. Going through these companies enables you to purchase the keys at half or even lower of the full Microsoft price.