Retool is really aimed at engineers who are building custom internal applications. For example, if you're Coinbase and looking to build a complex approval workflow for withdrawals above $100MM (with a custom UI depending on which Okta groups you're in, with custom approvals required depending on the KYC status of the customer, etc.), it's very very hard to do with either internal or forest (since the customizability and flexibility are much lower with both of them).
So: forest and internal are good if you want something quickly but have no intention of customizing it later. Retool takes longer to get started, but allows you to customize substantially more.
In the mid-term (i.e. 1 - 6 months' time) we want to make starting in Retool much easier. We will probably allow users to quickly generate admin panels (similar to internal or forest) with a few clicks on top of common datasources.