If you are a U.S. person or entity, failure to complete form W9 in a timely manner may subject you to backup withholding.
Backup withholding can amount to 28% of payments made to a US person. Foreign financial institutions have reporting requirements under FATCA. If you are a US person who fails to provide a timely W9, the institution will not be able to comply with FATCA and may close your accounts and refuse to continue to do business with you as a result.
For foreign persons and entities, failure to complete the W-8BEN or W-8BEN-E forms may subject you to pay extensive tax rates (up to 30%) or backup withholding.