transfer-gateway-example/truffle-ethereum/loom.pid

[pragmaticadam]

What is a purpose of this file?

[aaruel]

It corresponds to the process ID of the Loom dappchain after running ./transfer_gateway. It has no bearing on how the dappchain actually runs, rather for managing start and stop. As for it being in the transfer-gateway-example/truffle-ethereum/ directory, it was probably misplaced and never removed.