start of packet detection
Let us say you detected noise level power at -100dBm. You keep additional 10dB threshold to detect energy. Means if you detect >= -90dBm power, you try to detect preamble of Wifi packet. If you detect -95dBm power, you ignore it assuming it may not be a valid Wifi packet. This really works assuming you are very far from base station/access point(AP). Now suppose say you want a higher threshold so that you don't want too many false alarms (may be in order to save battery). So you raise the threshold to -60dBm. Here, if you are too far from AP, you may not trigger at all. If you are close to say within 10 meters, you start to detect energy more than -60dBm and try to detect Wifi packets. 

So, you can decide on threshold on how frequently you want to try to detect Wifi signal. 

If your threshold is too low, you may more frequently try to detect packets. Advantage - you can detect packets if you are far away from AP. Disadvantage- you may consume more power trying to detect packets. 

If you increase threshold, you will detect packets less frequently. Advantage - you can filter out signals which are weak and can use higher modulation rates. Disadvantage - you cannot move too far away from the AP.