For readability and sharing ETLs with co-workers, we (try to) create and thus order our relations in the same logical sense, where the highest level of detail is at the bottom. Like in Navision where we start with DW_Account = DW_Account and then work our way down to e.g. the line level.
Sometimes however you forgot to do this structurally and when trying to "interpret" your own or someone else's code this makes for difficult reading.
So it would be handy to be able to re-order relationship elements like you can re-order joins in a lookup field.