when dealing with big hosts or file systems you typically have an entire partition mounted on /usr . On this partition you keep application and services for your machine, in a production server you'll keep there for example BIND, SENDMAIL, APACHE or other stuff.
When you need to mount your os in single user mode for maintainance or you're facing an hw failure you need to have a fast bare bone booting system, you'll generally get a rid about stuff in /usr/ you only need configuration files (/etc), bash (or another shell), few services and nothing more. mounting /usr becomes slowly or as many of us do requires additional kernel params due to different file systems (XFS partition for example). So it's better to keep /usr where it is and mount it only when needed