If I create a flow that goes off as soon as I add a contact to a contact list, then I should be able to time all messages there from that moment.

New contact in the list = time X
X + 1 day: message 1
X + 3 days: Message 2
X + 5 days: Message 3

In the current system, I would have to count the days between each event in order to send a message exactly one week after day X.

If you want to know how crappy the current system is, then imagine the following process:

Customer logs in
Mail 1 after 5 minutes
Mail 2 after 3 days
Mail 3 after 7 days
Mail 4 after 14 days
Mail 5 after 21 days
Mail 6 after 28 days
Mail 7 after exactly 1 month