Back in the day, it was possible to download nightly snapshots of the in-development releases of MODX. Those were removed/broke at some point, and have just been restored to function.
The purpose of nightly builds is to make it easier to test what's being worked on. While it's always possible to build from git, nightlies are installed in the same way as an official release.
In lieu of official nightlies for MODX 3, Ivan Klimchuk launched a separate MODX 3 nightlies site a couple of months ago.
The next official release, MODX 2.7.2, is expected by next week.