You know how the BBC decided everyone needed an account before they could watch iPlayer, for reasons of personalisation? Channel 4 clearly feels the same way, and has announced that All 4 accounts will be mandatory from next year. The reason being personalising your adverts, and some other stuff.
The official reason is to offer a more personalised experience, tailored to the individual user with algorithms and whatnot. It just so happens that it enables the broadcaster to personalise the adverts people see in the process. That's going to happen on all All 4 services, except the ones on Sky and Virgin boxes - because Channel 4 doesn't actually control those apps.
Channel 4 has long been fond of personalising adverts, even going so far as to have adverts call people by their real name. Which isn't creepy at all. Will the new ads be that in your face? Maybe, maybe not, it's not clear right now. But analytics is terrifyingly clever, so if you log into a service and use it with great frequency they can build up a pretty accurate profile of who you are - even if you don't actually provide any of that information.