Stem length, orientation and height all influence comfort, bike handling and aerodynamics. A shorter stem implies a more responsive bike as well as less input from the rider needed to initiate turns. When seated, a longer stem helps lengthen the rider reach, putting the upper body in a lower more aerodynamics position. Moreover, a longer stem would normally put the rider lower for improved aerodynamics and farther ahead of the bottom bracket when sprinting a body position that generates more power. Conversely, a shorter stem would normally facilitate an upright rider position which is less stressful on the back shoulders, wrists and hands. Riding in this upright position can create more wind resistance which why professional riders is trying to stay as low as possible. The only way one can change stem length is with the new stem.
There are one or two adjustable length stems on the market, but they are usually not readily available and are relatively heavy.
A lot of stems are angled, which, based on how the stem is oriented on the bike, will raise or lower the handlebar position. Angling the stem down will lower the handlebar putting the rider upper body in a lower or more aggressive position. This can be more aerodynamics but harder on your body. Flipping the stem in order to raise the handlebars allows a straighter back and a more comfortable upright riding position.
The stem height can be adjusted as well by adding or subtracting spacers between the stem and the frame. Recreational riders usually prefer a higher handlebar position, wile professional cyclists tend to choose as low in aerodynamics as possible: factors such as terrain, style bike and rider fitness are all crucial factors in achieving the perfect body position through the manipulation of the stem using length orientation and height.