/usr/local was originally for local (ie host) software that wasn't in /usr/{bin,lib,sbin,slib...} or /{bin,lib,sbin,slib...}. Then GNU took over and made some

/usr/local was originally for local (ie host) software that wasn't in /usr/{bin,lib,sbin,slib...} or /{bin,lib,sbin,slib...}. Then GNU took over and made some opinionated choices about how those subdirectories were to be used, which is fine. However, because /usr/local is under /usr, it meant that it was more difficult to mount /usr as read-only or shared. There is also the issue of whether files under /usr/local should be owned by non-system users. A while back, the creation of /opt and /opt/{some-app} (and /var/opt/{some-app} became a way to avoid writing into /usr and also made it possible to mount applications with their code as read only with writable locations under /var. So I've taken writing to /usr/local as a packaging smell these days, purely to avoid writing into /usr.