It works when there is ongoing work, and fails when there is only ongoing availability.
A retainer for "access to me" quietly becomes a subscription the client cannot justify at renewal, because they cannot point to what it bought. A retainer attached to something that recurs — a monthly review, a reporting cycle, a standing decision they need help with — survives, because skipping it has a visible cost.
So before proposing one, ask what actually recurs in their business. If the honest answer is nothing, a retainer is the wrong shape and a series of defined projects is the right one.