首先，Linux的官方文档FHS对该目录的说明：http://www.linuxbase.org/betaspecs/fhs/fhs/ch04s09.html
The /usr/local hierarchy is for use by the system administrator when installing software locally. It needs to be safe from being overwritten when the system software is updated. It may be used for programs and data that are shareable amongst a group of hosts, but not found in /usr.
Locally installed software must be placed within /usr/local rather than /usr unless it is being installed to replace or upgrade software in /usr.