STP is atmospheric pressure and zero celsius temperature. The reason why we use it as standard (even though it is a bit frigid), is that it is easy to experimentally produce a zero C temperature, since all that must be ensured is two-phase coexistence of water and ice.
SATP is adjusted to a more comfortable temperature, more common in a laboratory. 25 C is used because it is an easy number to remember, and because it is the datum of most JANF tables of chemistry data. The 100 kPa as opposed to 101.325 kPa is used just to avoid the excessive numbers. AND because, 100 kPa is more common to more locations than the sea level 101.325 kPa. 100 kPa is approximately 100 meters of elevation above sea level, and an easy number to remember. 100 kPa is referred to as the bar, rather than as the atmosphere. I suppose you can consider 1 bar to be 1 "metric atmosphere".