A conductor performs a number of essential functions for an orchestra:

• Offering a pulse. You may think this is trivial if your experience with music is through rock bands and smaller ensembles, where the musicians can just listen to eachother. But keep in mind that a large orchestra takes up a lot of space, to the point where the speed of sound becomes a major barrier. A symphony orchestra easily spans an area of more than 30 meters, meaning people on the left hear what people on the right play 0.1s late. The 'beat' shown by the conductor arrives with the speed of light, keeping the musicians more together.
• Setting the tempo. Orchestral music tends to have more deliberate tempo changes and fermatas (points where a note is held longer than the implied beat would require).
• Controlling the dynamics and timbre that people play with. A lot of that tends to get ironed down during rehearsals, and sometimes it will be explicitly mentioned in the score, but it can be really helpful for players to reminded that, hey, perform this bit light and playful, and over here, be loud and solid. Especially in longer pieces.
• Cueing players when they have to start playing. In orchestral music, some instrument groups may have large swaths of time where they don't have to play anything (especially percussionists).

Generally the conductor is also responsible for the interpretation of the music. Orchestral scores don't tend to be black&white, and there are a lot of decisions to be made in how to approach a piece. The conductor is the one who makes those decisions.