Yes, I know that the sub-sonic/super sonic threshold is actually the speed of sound.
The speed of sound (in air) varies with air density, normally around 1070fps at sea level. I could give you the actual formula for figuring it out for yourself, but why bother? For most of us, that is those of us in the US that live below 5000'msl, it really doesn't matter all that much.
The major reason for 'sub-sonic' ammo is to reduce the sound signature of the bullet as it flies though the air. A super-sonic bullet actually makes a sonic boom, albeit a small one. That is the 'crack' one hears as a supersonic bullet flies by (a separate noise from the bang of firing).
Another reason for sub-sonic ammo is that an object flying through the air at sub sonic speeds has different aerodynamics than one at supersonic speeds. As the speed of an object increases, the shock wave pattern changes. When the object reaches the speed of sound, the shock waves detatch from the object. At this transition point, if the object has some minor imperfections at the rear edge, the force of the shockwave will be uneven, causing the object to deviate slightly in its flight. This transition effect also works in the reverse, like when a supersonic object (a bullet) slows down to the speed of sound.
This last reason is why target ammo for a .22LR is always subsonic. The heel of a .22LR bullet is very hard to make perfectly. A .22 high velocity round is just barely supersonic. As it slows and reaches the critical speed, the bullet will upset its flightpath slightly.
For those of us that are into Physics heavily, F=mv^2. Force = mass x velocity x velocity. The force needed to push the same bullet at a lower velocity is less than a faster one by a whole bunch. This means that we use less powder to make a subsonic round than a supersonic round for the same bullet weight.
Less powder = less noise at firing. Less noise means you can tolerate it better (especially in a confined space like indoors).
For every action there is an equal and opposite reaction. Less powder = Less force. So less powder = less recoil force. Less force (speed) on the bullet means less energy downrange. Less energy downrange means that less energy will be transferred to the backstop. Less energy transferred to the backstop means that at some point, you can stop the bullet with an indoor range type backstop without blowing holes through it! Indoors is neat when the weather gets too nasty to go outside to an outdoor range.