The "nightly.geany.org" is to provide those developing and testing Geany with a copy containing the daily merges so it is easier for the Geany develop team to have an up to date version. For the purpose of the Geany development version it is made to replace what has changed on the user system, not to be producing a full install of Geany since developers are likely to already have other changes they are experimenting and would not want that to overright their own development.
Geany does not produce a full Debian support install, that is performed by the Debian support crew meeting that systems rules. The same situation occurs for other systems, no installs are produced by Geany.