why does idle tab pay us?

It's a cryptocurrency mining app. It will try to use your resources and data allowance (if you are using mobile data) to mine 'Monero' (a cryptocurrency, something like Bitcoin). At least the site tells you what it is doing (there are some sites which don't) but it will only pay you a small percentage of the amount it earns when it's your electricity and data that it's using! Stay WELL away from such sites and make absolutely sure that they aren't opening a hidden pop-up screen to keep mining without your knowledge and acceptance!

I have never earned From offers yet