Some programs create temporary files and then leave them behind - they do not delete them. This can happen because the program crashed or the developer of the program simply forgot to add the code needed to delete the temporary files after the program is done with them. The temporary files left behind can accumulate over time and consume a lot of disk space.
Temporary files may be deleted manually. Operating systems may clear out the temporary directory on a reboot, and they may have "cleaner" scripts that remove files if they have not been accessed in a certain amount of time. Also, memory-based systems, such as tmpfs, inherently do not preserve files across a reboot.