Who can answer this English grammar question about the difference between past perfect&past simple"?

[Firstname]

Why is there a "had been" in this sentence - English grammar question.?
"I realized that social media would be transformational for businesses and decided to start a software company that would help Fortune 500 companies manage their marketing efforts using social media. Steve Garrity, who 【had been】 a computer lab partner of mine at Stanford, left his job at Microsoft to start that company, Hearsay Social, with me in 2009."
If I substitute "was" for "had been" then what's the meaning of the sentence and what's the difference between the sentence with "had been" and with "was"? Thanks in advance!
40 minutes ago - 4 days left to answer.
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Answer given by SirGeorg:
"Had been" is past perfect, "was" is past simple.

Steve, who HAD BEEN my partner, LEFT.
...................far past.......................past

It's like "By the time I arrived(past simple) everyone had already left(past perfect)"
It shows a sequence of things, if you used past simple ("was") it would still make sense but that sequence of things would be lost, it would mean that while he was your partner at Stanford he left to start a company with you, whilst if you used past perfect("had been") it would mean that he firstly used to work at Stanford, and after his work there was over, he joined you to form another company.
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Additional Details
SirGeorg..., do you mean if I substitute "had been" for "was", it will be wrong because people who read the sentence will think that when he was my partner and at the same time working at Microsoft, he left school and at the same time left the job to start a company with me?
Please answer the question to SirGeorg above. Thanks in advance!