Why does my shower feed do a long loop into my garage?
Hard to know without having them to ask. 

Possibly thinking of having a utility sink or clothes washer in the garage? My other thought was considering fitment of a pre-warming arrangement but that would only need the cold line (and a drain pump feeding the shower drain water to a heat exchanger on the cold line, for a shower not on an upper level. The pump energy being considerably less than is saved on water heating when this is done right.)