Used for temporary or easily recoverable data should the data be lost.
/scratch would usually be a separate partition on a separate, fast disk where reliability if of a lesser concern but performance a boon.
Files and applications to be considered for /scratch:
- distribution packaging (e.g. Gentoo's portage tree).
- off-tree compile directory (the build/ directory in software source tree).
- blender 3D bake files.
- new/ and tmp/ directory in a mailbox directory, as opposed to the cur/ directory which permanently stores the emails.