for those of us just starting out on linux...
would you care to explain why?
The each directory in the basic Linux directory structure has a 'purpose', to help things work between distributions and be semi-standardized.
/bin is for essential utils needed during boot, before the network may be up.
/sbin is for admin utils that are needed before the network is up
/usr/bin is for day to day use tools that aren't needed until /usr is mounted and the network is up
/usr/sbin/ is for admin utils that aren't needed until /usr is mounted and the network is up
lib directories are generally for the same purpose as the bin directories except they have shared libraries (i.e. dlls) instead of executables
/usr/local is for programs compiled just for the local host.
/etc is for configuration files
/var is for frequently changing files, mostly logs, mail and news spools.
/home is for user home directories
/proc is for process information, in Linux it's been expanded to hold a lot of kernel variable information
/tmp is for temporary files, this can be put on another partition to avoid filling up the / partition, or if you have the memory it could be a ramdisk or virtual memory filesystem.
There's documentation in the LSB that probably explains things better than I can.
A lot of the reasoning takes into account situations that a lot of people won't even think of, like mounting /usr and/or /home over the network via NFS is popular for a lot of reasons. Hell if you really want you can mount the whole root filesystem via NFS and not have a local hard disk.