Why bother change it when it works perfectly during the flight ?
Because when airplanes are flying below 10,000ft (the start of cruising altitude), they’re generally flying to takeoff or land. Meaning there’s probably plenty of other planes in the area either converging in one area or departing from an area and with so many planes in a certain about if 3-dimensional space, and that they can’t necessarily see each other in that space, air traffic controllers are responsible for controlling and helping them navigate that space to ensure that they don’t collide in air or on the ground.
Above 10,000ft, planes are headed to cruising altitude and are far enough away from the hub where there’s a high density of planes in the space that they can generally navigate and watch out for each other using their own instruments and plane-to-plane communications. There are protocols for how pilots communicate with each other and engage with each other to avoid collision
Actually, the crossover is FL180 (Flight Level One Eight Zero, or 18,000 feet).
Altimeter settings are based on ground observations. The higher you are, the greater the potential for error exists between the altimeter and the ground station.
With their altimeters all set to standard pressure, airplanes in the same part of the sky can more assuredly maintain vertical separation.
Above FL180, precisely maintaining a particular true altitude is less important than not hitting other airplanes.
A secondary factor is performance. Calculating performance metrics such as speed or fuel consumption depends on pressure altitude. Performance data assumes a pressure altitude, to which a conversion is applied. If you already know your pressure altitude (because an altimeter set to standard pressure shows pressure altitude) then you can skip the conversion.
Using U.S. rules, you use the local altimeter setting up to 18k feet MSL. Above that, you are in Class A airspace where you use the standard altimeter setting of 29.92. Additionally, altitude at this point is no longer feet MSL but Flight Level (FL), roughly corresponding to thousands of feet, so 20k’ feet would be referred to as FL200. The reason for this is that Air Route Traffic Control Centers (ARTCC) control very large regions where there can be large variations in the local altimeter setting. Also, at these high altitudes, obstacle clearance is of secondary concern to traffic avoidance. So, having everyone on the same setting means that you can just assign a flight level and not worry about constant adjustment.
Once you descend out of the Class A airspace obstacles start to become a concern, and their elevations are given in feet MSL. Instead of having everyone do the mental math to convert their FL to actual altitude, we just use the local setting.
Having everyone in the air on the same setting helps ensure that everyone flying through a particular area is on the same setting (and not constantly having to change altimeter settings), so vertical separation between planes isn’t affected by varying altimeter settings (in other words, what matters is your position compared only to other planes, which is why you want everyone measuring altitude the same way)
At lower altitudes, knowing your exact altitude in comparison to ground level and terrain/obstacles can be important