Great question and from what I have read, they are pretty popular in Buy Side firms, Market Research firms and Management Consultants. Unsure, in what ways do corporates benefit from them.

Feel free to DM me. I've used them extensively as a client and also consulted few times as an expert

I've used a variety in consulting capacity in the past; be very clear on what you're trying to achieve, backgrounds that will be helpful, and your expectations/needs. Also budget for a handful of conversations because some percentage are always duds - i.e. people oversell their capabilities and a few questions in it becomes clear, or they clearly have an axe to grind with a former employer and it's difficult to tease out the real truth from subjective perspectives without enough data points. You might have one expert who gives you everything you need, but it may not be the one you expected.

Do they offer one-time engagements/consultations or do you need the “subscription” as well?

I can't remember which ones I worked with in the past as each probably has a slightly different model, but yes there are one time engagements that you can pay for vs subscription. I've definitely done one time engagements where you just pay for the time for the experts they put you in touch with. Markup and price per hour is probably higher, but it's available.

Most dont offer ala-carte. Quite expensive.