Hello @DuncanHornby ,
The other comment is correct. This is a new way for custom tool developers to define Parameter Descriptions. So, when you open the tool, the text entered there should display when you hover on the i next to the parameter. However, the feature is not finalized.
I can confirm that the parameter description is discarded when you click OK to close the Tool Properties dialog. For now, you can continue using the metadata to edit the parameter descriptions using the method you outlined above. And we plan to update the Parameter Description option on the Tool Properties dialog in 3.4 or a feature release.