dupe I believe that if you skip that form, they will by default withhold 30% for taxation in the US.
Obviously, I'm not 100% sure about that, it would be great to get more info from StellarOrg on that.
Edit: the instructions are pretty clear.
Failure to provide a Form W-8BEN when requested may lead to withholding at the foreign-person withholding rate of 30% or the backup withholding rate under section 3406.