system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why does a sniper need a spotter, when both of them are using scopes to look at the target? And how does the spotter know the wind speed if there are no moving objects like trees or a flag?
gpt
They're both using devices that give them that information. 

A spotter is used because while a sniper is certainly capable of gathering all that information and calculating it, having someone to feed him/her numbers speeds up the calculation of a long shot significantly. 

That's especially important if lives are contingent on the sniper taking down a target quickly. 

As for wind speed, etc., an electronic anemometer has been available for almost 2 decades now. 

A spotter gets to position this and other devices and convey the data to the sniper in a single package. And the spotter isn't in the same place, so assuming good communication, you have a much wider "field of vision" based on the spotter's reports. 

Wind speed, direction, presence of glass, movement, possible civilians nearby, etc. are all concerns for a sniper. A spotter alleviates many of those concerns and allows the sniper to focus on accuracy.