The imagined ensemble may seem a pointless delusion, but orchestral hornplaying, especially in its soloistic capacity, is largely an exercise in preparation and panic. Unlike the trumpets we at least get to play nearly half the time, which helps to keep us in shape, but those critical solo passages still come and go all to quickly. Which is why the orchestra of the mind plays such an important role in solitary preparation. In essence, we have to learn to mentally transport ourselves from the practice room to the performance hall, supplying the ensemble, audience, and acoustics by power of will. Only in this way can we begin to gain experience and knowledge of our literature in its contextual, unexcerpted form.
Modern technology can help a bit here, but only a bit.
Symphonic recordings can help with establishing ones place and relationship to the other instruments. Metronomes can encourage rhythmic consistency. But ultimately, we must be our own conductor, ensemble, audience, and critic. Of course playing from ones seat at the back of the orchestra and to a live audience will never be the same as the practice room illusion. Whenever I played something wrong, or simply had a high note fall apart on me rather than speaking, I would look into that little hole in the mouthpiece and try to understand what I had done to offend its deities.