The vast majority of what a conductor (normally the music director) does is not the actual conducting of the orchestra at a performance. At the performance, he (and increasingly it is also she) is responsible for determining the tempos and dynamics of the orchestra as well as the overall line of pieces being played. Most important, the conductor keeps the orchestra together, from the beginning to the end of the piece. The conductor will know the piece better than any of the individual musicians in the orchestra, who are basically responsible for their own parts. Many will essentially know every note each of the 100 members of the orchestra is playing, and if anyone plays a wrong note or enters at the wrong time. Different conductors may have very different views on how a piece is to be played, even to the degree of how the orchestra is seated. For example some conductors prefer for the second violins to sit next to the first violins, and some prefer the second violins to sit on the opposite side of the orchestra from the firsts.
Behind the scenes, among the many things a conductor/music director does is choose the pieces that will be played, taking into consideration the mix of old and new, well known vs unknown, strengths and weaknesses of the orchestra, any soloists who may be engaged, and how to balance all of the considerations with the economics of the orchestra's budget. Assisting with money raising is another important role for a good MD. Most music directors who have a decently long tenure, will be responsible for hiring and promoting, and sometime demoting and nudging out members of the orchestra. The conductors, both MDs and guests will also rehearse the orchestra - usually 2 or 3 time, so that by the time of the performance, the orchestra knows what to expect from the conductor.
Conductors have their own strengths and weaknesses, both in terms of repertoire and management skills. Most last a few years (8 years is a pretty long term for a major orchestra, 3 is a very short term) as MD of a major orchestra. Those who last a really long time are very rare - Michael Tilson Thomas (MTT) of the SFSO is approaching 25 years as MD (he will retire in 2020 at age 75). Other really long term MDs of major US orchestras have been Seiji Ozawa in Boston and Eugene Ormandy in Philadelphia. I think that is about all of them. Al most all conductors who are MDs also guest conduct. We have heard MTT conduct in London where he has been a guest conductor of the London Symphony for many years. There a large number of young conductors who do not have an MD position and they basically spend their lives looking for an MD position, apprenticing with a major orchestra (as assistant or associate conductor) and guest conducting.
Larry