Consider a professional orchestra with a fairly wide repertoire. Obviously it will need players of all of the strings pretty much all of the time. However, for many of the other instruments, the need will vary a lot.
Consider the clarinet next. Many older pieces will need none. One, two, or three will be commonly required. Some pieces, e.g. Stravinsky's Rite of Spring, will need five. Does the orchestra retain five clarinet players but some sit idle a lot? Do they need to engage individuals to boost the numbers when required?
How about rarer instruments (in the classical repertoire). The saxophone appears occasionally but not often. So, I doubt that there will be a retained saxophone specialist. A clarinetist might double on the saxophone but pieces that need the saxophone often need a clarinet as well so there may be no spare retained clarinettist. Similar comments apply to other occasional instruments e.g. oboe d'amore and larger flutes.
It is not obvious what a harp player might double on but many pieces require nohe whereas others require one or two.