Moderation, identity services, profile services, matchmaking (invites, joins, statuses, friends), communication services (voice, text, images, etc.), misc. social services (looking for group, trophy/achievements), platform upgrades if you want to count that, and the customer support.
The business model of at least Sony and Microsoft is to sell you a console almost at cost and then monetize on you through accessories, publisher royalties, services and now digital store-front cuts. Running (maintain too) and developing the services/features that enable the experience cost a lot of money and they could just eat that cost like they eat the R&D cost of their console but they choose not to. They aren't simply charging you to access the Internet (they aren't even technically doing that because you can use the Internet), they are charging you to access functionality of games that are mandated to be highly integrated with services that cost them millions to operate.
They don't do it on PC because they can't do it effectively due to the inherent nature of the platform and PC distribution platforms do a lot of cost cutting. Most if not all the PC stores use technologies that let them do things fast/cheap and they don't care about offering every features because users can just run multiple applications. Services like Steam also cut cost by having limited customer support options. These store fronts also used to take huge cut of around 30% of every sale whereas I believe console manufacturers get around 12% for traditional retail.
I think that as console gaming goes full digital (or closer to full) there's less of an excuse for MS, Sony to charge for online as their model drastically changes to greatly benefit them so that eating the costs of the services is what they should do.