Just guessing about the compressor but gas fed into the sales...

Just guessing about the compressor but gas fed into the sales line must be at the sales line pressure (there's probably a non-return valve inserted so that you don't swallow other people's production but you wouldn't be able to sell the gas until pressure builds up).

Though well head pressure is high, flow will be restricted (choked down) to protect the well, certainly initially.

There is a huge volume difference between vapour and liquid, so, when the condensates and NGLs liquify (if the NGLs are being separated) there will be a huge drop in pressure.

That is possibly why they need the compressor.