- They need to prove that "you" accessed the service, not somebody claiming to be you, somebody with the same full name and rough location as you, etc.

- Websites are often pretty bad at actually requiring ToS acceptance to register. The fact that you've accessed the service doesn't necessarily imply you've accepted anything.

- Even if registration is ironclad, accessing the service won't prove you agreed to any particular version of the ToS. The ever popular amend-at-will clauses never hold up in court, so you really do need to know which version was agreed to.

- ToS are often presented coercively. Maybe you've already signed a lease and moved in, but to actually pay your rent you need to accept an additional one or more third-party ToS because the landlord doesn't accept cash or checks. Maybe you've already paid for your vehicle registration, and after the cash is removed from your account you're presented with additional terms that need to be agreed to in order to receive your tags. Even if you've agreed to some specific contract, that kind of coercion can invalidate the additional terms, even though the party whose ToS you agreed to might not have known about the coercion.

ToSes are mostly useless. They generally contain a line that indicates the terms can change at any time and you accept them by continuing to use the service.