The nightly builds allow people to test the latest developments and bug fixes sooner than the prerelease or stable versions. The goal is to help people catch new crashes and regressions before they reach the prereleases or stable versions. For a more detailed description see Comparison of stable, prerelease, and nightly builds
A nightly build can run side-by-side with other versions of MuseScore on the same computer. If you change the preferences in the nightly build it does not affect your preferences in the prerelease or stable versions of MuseScore. The nightly builds use a cymbal logo to visually differentiate them from normal versions of MuseScore. The cymbal is a play on the fact that nightly builds are likely to crash. If this sounds scary to you then I recommend using the latest stable release from www.MuseScore.org