This answer states that Space Shuttle's External Tank (ET) was fuelled (this process is known as Tanking) through the Orbiter's main engine plumbing system with the Tail Service Masts (TSM). One delivered LH2 and the other delivered LOX.
For reference, here's a picture of the TSM fuelling Space Shuttle Atlantis.
However why not just directly fuel the ET instead of the fuel and oxidizer going through the orbiter. It seems like the orbiter's fuel plumbing system would be a lot more complex with fuel going from the TSM, into the orbiter, and then into the ET. Not only that, but the LOX had to be pumped up to where the LOX tank was located (in the upper half of the ET) whereas if it was directly fuelled into the ET, the pipes can be pumped upward in the tower possibly saving weight on the ET by not having a LOX feed-line.
Question: So what are the advantages or reasons that got NASA to fuel the ET through the orbiter instead of directly fuelling the ET?