Conversation so far:
what is the importance of Tranquility / Node 3?
Tranquility, also known as Node 3, is a module of the International Space Station.
who came up with the design for Tranquility?
ESA and the Italian Space Agency had Tranquility built by Thales Alenia Space.
is there anything else interesting about the design of Tranquility?
Tranquility provides six berthing locations with power, data and commanding, thermal and environmental control, and crew access for more attached habitable volumes or for crew transportation vehicles or stowage.

Question: why did NASA leave the MPLM permanently attached to the ISS?
Self-contained rewrite: why did NASA leave the MPLM permanently attached to the ISS?