Every fractional arrangement has to answer one question: how do you charge for judgment? The default answer — hourly — quietly breaks the entire relationship, and here's the mechanism.
The second you're paying me by the hour, you start rationing me. You sit on a decision for three days because pinging me costs money. You don't mention the thing nagging you because it might be nothing. And the decisions that matter most are exactly the ones that feel too small to "spend an hour on."
A retainer aligns the incentives
On a flat monthly fee the calculus flips. Call me whenever. The 6am idea about your architecture, the gut-check on a hire, the "is this vendor ripping me off" — all in scope, because you're not buying time, you're buying access to judgment.
It keeps me honest in the other direction too. I can't pad a timesheet. The only way I keep the retainer is by being obviously worth more than it costs, every month, on a month-to-month basis you can cancel. That's a far better deal than a stack of invoices you can't evaluate.
Roughly 80% of the market uses retainers for exactly this reason. The ones still selling hours are usually selling hours because that's all they have to sell.