Windows generates a unique advertising ID for each user on a device, which app developers and advertising networks can use to provide more relevant advertising in apps. When the advertising ID is enabled, apps can access and use it in much the same way that websites can access and use a unique identifier stored in a cookie. Thus, app developers (and the advertising networks they work with) can use your advertising ID to provide more relevant advertising and other personalized experiences across their apps.
Turning the advertising ID off will not reduce the number of ads you see, but it may mean that ads are less interesting and relevant to you. Turning it back on will reset the advertising ID.
Some websites may have their content available in different languages. Windows can share information about your preferred language list with websites so that they can have the opportunity to respect your language preferences without you having to independently set them for each site.
Windows can personalize your Start menu based on the apps that you launch. This allows you to quickly have access to your list of Most used apps both in the Start menu and when you search your device.