My "ah ha" moment for why they're shitting up their OS with ads was when I discovered my uncle had managed to follow their prompts to the point of paying for One Drive, but had no idea what it was or how to use it (in fact, he was confused how some of his files were becoming "lost"). He just thought it was "something he needed".

[EDIT] which is to say, they're joining Google (ads inline with results—watch a non-geek use Google some time) and Amazon (just... the whole store) with a scamming-the-unsaavy business model.