Conductors are very useful, they function as a metronome, as an audio engineer, and as someone to queue people in. Many pieces do not have a drummer, so knowing when to come in is very important. A good conductor will practice with an orchestra for weeks or months before a piece is ready for performance. A conductor also adds a human element, they can slow down a piece or speed it up, increase the volume or lower it, and the spot where they stand offers a great place for knowing how loud each instrument section should be (this is critical in a full orchestra with wind instruments and string instruments, trumpets are much louder than cellos for example). Some conductors are very strict, and some conductors could step away and the orchestra would play itself. The best conductors are somewhere in the middle.

If you find this at all interesting I would encourage you to find a way to watch an orchestra rehearse, as watching rehearsal is a great way to understand how a conductor interacts with an orchestra. It's probably very easy if you have kids, just go watch them play in school. It's a bit harder if you don't have kids though, as options for adults are much more scarce, but sometimes there are bands in neighborhoods you can go find.

Source: Years of band practice in school, sometimes with a full orchestra.