Your GC (General Contractor) needs your W-9 to report payments to the IRS. If they pay you $600 or more in a year, they must file a 1099 with your taxpayer information.
This does NOT mean taxes are withheld from your check. As a subcontractor, you receive your full payment and are responsible for paying your own taxes quarterly.
Your SSN or EIN on this form is only shared with the GC — it is not sent to the IRS directly.