Aside from the fact that someone has to lead the arrangement (it can be done by consensus or pre set as well) that a conductor was needed simply because of this

Aside from the fact that someone has to lead the arrangement (it can be done by consensus or pre set as well) that a conductor was needed simply because of this distance between the players. 40ft gives a delay of about 40ms, which is a lot in music. Add in the echoes and reverberations and the link between the musicians gets very loose. Musicians can get used to playing with the various delays inherent in sound propagation through various mediums, but need to practice. Most of the players in orchestra are not hearing anything like what the audience hears, so having someone front and center is essential to pulling it all together.