I'm not going to try to address the "Whys" (since I'm not competent to do so) but you may find the following useful. This is from the European Space Agency. There's also a really nice graphic to go with it on the site:
L-band (1-2 GHz) Global Positioning System (GPS) carriers and also
satellite mobile phones, such as Iridium; Inmarsat providing
communications at sea, land and air; WorldSpace satellite radio.
S-band (2–4 GHz) Weather radar, surface ship radar, and some
communications satellites, especially those of NASA for communication
with ISS and Space Shuttle. In May 2009, Inmarsat and Solaris mobile
(a joint venture between Eutelsat and Astra) were awarded each a 2×15
MHz portion of the S-band by the European Commission.
C-band (4–8 GHz) Primarily used for satellite communications, for
full-time satellite TV networks or raw satellite feeds. Commonly used
in areas that are subject to tropical rainfall, since it is less
susceptible to rainfade than Ku band (the original Telstar satellite
had a transponder operating in this band, used to relay the first live
transatlantic TV signal in 1962).
X-band (8–12 GHz) Primarily used by the military. Used in radar
applications including continuous-wave, pulsed, single-polarisation,
dual- polarisation, synthetic aperture radar and phased arrays. X-band
radar frequency sub-bands are used in civil, military and government
institutions for weather monitoring, air traffic control, maritime
vessel traffic control, defence tracking and vehicle speed detection
for law enforcement.
Ku-band (12–18 GHz) Used for satellite communications. In Europe,
Ku-band downlink is used from 10.7 GHz to 12.75 GHz for direct
broadcast satellite services, such as Astra.
Ka-band (26–40 GHz) Communications satellites, uplink in either the
27.5 GHz and 31 GHz bands, and high-resolution, close-range targeting radars on military aircraft.