What is the difference between /opt and /usr/local?

According to the [Filesystem Hierarchy Standard](https://www.pathname.com/fhs/pub/fhs-2.3.html) , /opt is for "the installation of add-on application software packages". /usr/local is "for use by the system administrator when installing software locally". These use cases seem pretty similar. Software not included with distributions usually is configured by default to install in either /usr/local or /opt with no particular rhyme or reason as to which they chose. Is there some difference I'm missing, or do both do the same thing, but exist for historical reasons?