This request has been asked quite a lot on the developers forum.
Use case is to have Alexa control systems with user-definable entities, e.g. a home monitoring system with user-created monitored channels.
For example, I'm monitoring outside temperature, power consumption and power generation. Those values would be candidates for slot type values of "channel".
Alexa API is lacking the ability to dynamically populate slot type values depending on external data.
That would be a GREAT addition, opening quite a number of additional use cases.