Its done to remove the ECU's control of the dump valve. The ECU on your Leon uses the N75 (boost pressure control valve) and the dump valve in combination to control boost pressure. There is no need for it to do both and you often find that it opens the dump valve for a fraction of a second when it doesn't need to. This robs you of power and it takes longer for the boost to meet what the ECU is actually asking for. The N75 is perfectly capable of doing the job by itself as it does on nearly every other turbocharged car. The AUQ engine has dynamic MAP sensing and overboost fuel cutoff that will stop a dangerous situation if the wastegate were to jam shut or the N75 valve was to fail, there is absolutely no need for the N249.
Bypassing the N249 means that you are running the vacuum feed to the dump valve directly off the intake manifold so that it can't open until you actually lift your foot from the throttle pedal.
If you think you're down on power, I'd be checking all the hoses coming off the manifold for splits and tears, especially the crankcase ventilation hoses on the underside of it. Either that or log the fuel trims and requested vs actual boost on Vag-Com, these will show if you have a boost leak somewhere.
Not exactly basic but its what you need to know to have a proper understanding of the system. In short, the N249 allows the ECU to open the dump valve. Its not required and the ECU isn't very good at it. The boost control setup works better with the N249 bypassed.