Collecting SCM data during code analysis unlocks a number of SonarQube features:
code annotation (blame data) in the Code Viewer
SCM-driven detection of new code (to help with Fixing the Water Leak). Without SCM data, SonarQube determines new code using analysis dates (to timestamp modification of lines).
Turning it on/off
SCM integration requires support for your individual SCM provider. Git and SVN are supported by default. For other SCM providers, see the Plugin Library.
Then, if need be, you can toggle it off at global/project level via administration settings.