At startups we've worked at/for in the past, they implement a Wait List before the App is fully built
so that marketing efforts can commence in parallel to development.

Examples

There's no shortage of examples on Dribbble: https://dribbble.com/tags/waitlist
https://dribbble.com/shots/20470789-AlignUI-Design-System-Waitlist-Page

Context

As an Engineer I've always thought that all efforts should be focussed on the product, 🧑‍💻
and that artificial wait lists are a bit of an anti-pattern. 👎
Why make people wait if the product is ready for them to try it now? 🤷‍♂️

"Waiting lists have been associated with negative psychological and physiological responses such as anxiety and stress; more uncertain, unexpected and longer waits lead to further aggravation of these symptoms" ~ https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0265542

I still maintain that forcing people to wait to use a product is lame as it destroys momentum.
The only case I would consider it would be if we didn't already have an MVP
or we thought our MVP was too poorly executed and we had an unmissable marketing opportunity
and could afford to worry about post-event/opportunity "conversion rate" ...

Possibly Valid Use/Situations

Imagine that you as the founder/product-owner of the App want to review each signup
and send a personal message welcoming the person to the App.
Feel free to correct me, but I don't think we want to personalise the App onboarding experience. 💭

The only other reason for rate-limiting and waitlisting people is a technical bottleneck.
And this one is real: GMail only allows 250 outbound emails to be sent per day. 📧
Once we reach that limit, we either need more Google Accounts or a rate-limit.

If we had 250 sign-ups per day we'd have very good problems!