…it’s the bassist’s JOB.
Stupid little word, who gave you permission to go on vacation :dubious:
I’ve been turning this over in my mind a bit more. Once upon a time, when orchestras were smaller, and usually the “house band” of some major aristocrat, it was common for performances to be directed from the keyboard, or by the leader/concertmaster, i.e., from someone performing with them (albeit the designated musical director, as for example Haydn), thus, not too different from chamber music ensembles then and now. But with the development of romantic music, freelance composer/conductors, and the expansion of orchestras to match the ambitions of the time (roughly starting with Beethoven), it all got too big to be handled the same way. I’m reminded of this because it has been much remarked on here that Joshua Bell is about to perform Beethoven’s fourth symphony at the BBC Proms, from the leader’s desk.
Noting the point above about not being able to hear what everyone else is doing in a big chorus, it seems obvious that where a small chamber ensemble can work to nothing but sound cues from each other, a big orchestra can’t - and maybe with a hundred musicians or more on stage, are they so far apart that there’s an appreciable delay in someone picking up a sound from the other side of the orchestra? Don’t know what the optimum number (or distance) would be, but it would be much smaller than a modern symphony orchestra.
Anyone who wants to get a good overview of the development of orchestral music would probably find the BBC series “Symphony” interesting. It includes a lot of clips of both conducting and performance, which give a good idea of what the conductor (in this case Mark Elder) actually does.
https://www.youtube.com/watch?v=uNjMN0yIPXo&index=1&list=PL72jhKwankOip0AeDp7qypAgbQdZZ6w4I
However, as noted, virtually all of the time this is something that the conductor will sort out in rehearsals; if he or she wants the bassoons to play forte where the score is marked mezzopiano, the conductor is highly unlikely to suddenly spring it on the bassoons on concert night. Yes, they will often gesture for a little more or a little less but by the concert the musicians ought to have a pretty good idea what the conductor’s expectations are.
Watch the conductor in this clip. The piece is Haendel’s Music for the Royal Fireworks, played on period instruments. The tempo is a bit faster than most interpretations, but it is not incorrect. This is probably as close as we’ll ever get to hearing it the way the composer did.
With a different conductor, it would undoubtedly sound very different.
I saw Frank Zappa in concert once, and he actually did conduct the band during a few numbers.
I know we’re mostly talking about symphony music here, but as a music director/keyboardist for musical theatre/Broadway-type stuff, I just wanted to add that the conductor (usually the same as MD/Key 1, these days) is essential to know when to start playing. The rest of the band is usually in the pit, which means they can’t see the stage. In addition to teaching the vocals to the cast, prepping the band in rehearsals, and playing during the show, they also give a warning motion to the band a few lines before the cue line, then need to time the count-in (if needed) with the end of the cue line, so there’s not an awkward lag before the music starts.
It can be a high-pressure situation, depending on the complexity of the show. But it’s nice to actually be able to see the stage!
And, if there’s untimed/unmetered dialogue that happens over the music, they make sure that the music and dialogue meet at the appropriate place.
Of course he did: Frank was a composer, not a rock star.
…and in some of his music, “where the hell are we anyway” is a question that frequently needs answering. 
AIUI, one of the main purposes of a conductor is to reset/rein-things-back-in when things are starting to go haphazard or awry with the orchestra, such as musicians not being fully in sync or losing track of things. The conductor keeps the pace, keeps things going on the straight and narrow.
Anyone still interested in this question might be interested in the BBC Radio3 series The Silent Musician - I believe they’re available to people outside the UK, but just occasionally rights issues might get in the way: