One list you both actually open
Add something on the way home and the other person sees it immediately.
Who it is for
For a household that runs on "did you remember to"
The shopping list, the thing the school needs by Thursday, the appointment somebody has to book. It is a lot to hold, and it is usually one person holding it.
Ask in the task
"Which brand?" belongs on the item, not in a separate message.
Some of it is still yours
A private task inside the shared list, for the surprise or the thing you have not decided yet.
Free
For the whole household.
It works for two people as well as it works for a team, which is the point.
Also for: small teams, freelancers.