Metal pipes expand when heated up and contract when cooled down. If you have a long right pipe there will be a lot of force on it and it can result in crack pipes from stretching or bending.
Look up sun kink and you see what can happen to the railroad for the same reason. Historically the way it was solved with a small gap between each piece of track
Bridges have expansion joins where they can expand and contract without cracking.
For a gas pipe, you can just have a gap, the gas would escape. The way it is solved for gas pipes is the U bend. The people there can bend in a safe way 90 degrees to the direction it points. Just take a drinking straw or any pipe and compare how it behaves if you bend it compared to stretch it out or compress it. It is quite clear why you like the begin at 90 degrees in the direction of the pipe.
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