These typically take place when no one is likely to be working in the office so that there are no changes to the source code during the build.
In contrast, continuous integration environments automatically rebuild the project whenever changes are checked in – often several times a day – and provide more immediate feedback; however, they do not necessarily include nightly builds.
CI jobs are often run on isolated virtual machines, and typically include automated testing as well.
This is typically due to additional developer-specific changes that were either not checked in, or (in the case of environment variables, etc.)
Neutral builds are important for software development processes running at high loads with short schedules (see extreme programming, startup).