Structure pipes do not move any fluid, power or item, but they can be useful to transmit pipe wire signals from one type of pipe to another type, i.e. an iron AND gate on a waterproof pipe transmitting a red pipe wire signal, which is carried by a structure pipe with a red pipe wire, to a golden OR gate on a regular pipe.
Although they can't move any items, they are considered a valid inventory by the Factorization Router. So if you want to connect your Router to a set of machines a bit further away, you can use a Structure pipe to connect the inventories between them.