Why does my client need a US Tax ID?

SussexAccountancy

I have a graphic designer client, who does small amounts of work with US clients. She is a UK resident, based in the UK, doing work for a particular NY client. The work totals around $2000 per annum. She invoices them (ie: she is not an employee), she never visits the offices, communicates by electronic means only. As usual, she has filled out a W-8BEN form, which certifies exemption from US tax, but she is being asked by this particular NY client to get a US TIN (Taxpayer Identification NUmber) Surely she does not need a TIN? If she does get one, presumably this could make her liable to filling in a US tax return, as well as complicate her UK tax return?

Does anyone have any experience of this, as the client is putting pressure on to organise this, and my client does not want to upset them, but I can't see that this is necessary if the W-8BEN form is filled in

Thanks SA