Why to write for instruments that can't be heard in the mix anyway?

Any learning composer will sooner or later get to the point of asking themselves why to put effort into writing parts for instruments in passages where they will be inaudible in the mix anyway.

In fact, every brass section playing a fortissimo with a "film score attitude" will drown out almost any other instrument except for maybe percussion, high flutes/piccolo and high strings so the question why to actually spend time and effort writing a part for instance for the 2nd Clarinet is definitely valid.

One general concept with orchestra (recordings) that differs quite heavily from for example a band recording is something that could be called desired intransparency. When mixing a band recording one usually carefully creates space in the frequency range to place all instruments properly in the mix and avoid anybody from drowning. With orchestra recordings however, due to the sheer amount of instruments it is impossible to make them all audible in the mix but even more it is not desirable to do so.

A great orchestra led by a great conductor very often creates such a homogenous overall sound that it feels like "one big instrument". In fact, the entire "grandness" and symphonic quality in sound that an orchestra creates is based on a mold of heterogenous sound colours that as an ensemble create this richness in sound.

Consequentially, even instruments that seem to drown under other instruments create part of this ensemble effect. It might in fact be debatable if there's a point in having the 2nd Clarinet mentioned above play anything in a "wall of sound" fortissimo brass passage however in more "civilized" passages, in spite of not being able to single out this colour consciously, its absence would be very much noticeable. 

So there definitely is a point in writing for instruments even when you will not be able to consciously hear them later on in the recording. The same thing applies to sample productions. While it will never reach the symphonic homogeneity of a real orchestra, you can approximate a similar effect in mockups by massaging the sound with parts for inaudible yet "perceivable" instruments.