purpose of xp extensions

Hi: for many years I just follow the document: create certificate with xp extensions and use group policy to deploy the CA. I never really understand what is. now I have several stupid questions: 1. is the "xp" extensions means windows xp or something? 2. is xp extensions only useful if we want client to verify server certificate? 3. if we use certificate like let's encrypt without xp extensions. what function do we miss? I know it is not very secure to use public CA, but it seems easier when deal with mobile devices bring by users. they just want to access wifi with their active directory username/password. thanks a lot for help!!