Ive reas most of the comments and the one that makes the most sense is the ones saying to group arrows together to stop people dodging them. I want to agree with that fully but i also doubt i could ever see a arrow flying towards my face. I do archery and yeah. But one that i havent see. Yet would be. One arrow is enough to kill someone.. but its also not enough to kill someone. Firing all at once can put multiple arrows into the same person ensuring that he wont get back up. I also believe it could be down to rhythm. Most amriest use rythm from martching to rowing. Its more frightening seeing a entire army work as a team than a bunch of people each doing their own thing.

I could be wrong but thats what makes the most sense to me. I could have went into more detail but chose not to because there is alot to read

I've read most of the comments and the ones that make the most sense are the ones saying to group arrows together to stop people dodging them. I want to agree with that fully but I also doubt I could ever see a arrow flying towards my face. I do archery and yeah. 

One arrow is enough to kill someone... but its also not enough to kill someone. Firing all at once can put multiple arrows into the same person ensuring that he wont get back up. I also believe it could be down to rhythm. Most armies use rhythm from marching to rowing. Its more frightening seeing a entire army work as a team than a bunch of people each doing their own thing.

I could be wrong but thats what makes the most sense to me. I could have gone into more detail but chose not to because there is a lot to read.