A choke valve - provided it is correctly set up (ie in critical flow) - isolates the pressures on the upstream side from pressure changes on the downstream side. So, in oil & gas production, you want to isolate the bottom hole and wellhead pressures in the well from pressure changes in the downstream production train, which in turn allows you to produce at a constant rate from the reservoir. This avoids coning, cusping, reservoir damage and all sorts of other bad things, irregardless of any short-lived (and small) pressure changes on the production train side.
In essence it's the way to control flow from the reservoir, and choke optimisation is a big part of a production engineer's job.
