Fireplace insert install

Hello, I have an fireplace insert I purchased off of craigslist, and the previous owner included the stainless steel liner, plus a bunch of stove pipe. I am installing into an existing brick chimney. I believe the previous owner also had a brick chimney this was installed in. I don't plan on doing the install myself, so maybe this is the wrong website to be on, but... I had a contractor over to give me a quote for install, and he said that the stove pipe was unnecessary in a brick chimney - that only the stainless steel liner was necessary. Who is right? What is the purpose of ever installing the liner inside the stove pipe? Thanks!