Tenant asking landlord for W9

[Lee L.]

One of my tenants is asking me for a W9. Ever since they've moved in, they've been paying their rent in 3 checks: a personal check, a check from their first business, and a check from their second business. This has not been a problem so far. They're great tenants who I intend to keep. They're creative types and do freelance from home for a living.

Are there any reasons why I shouldn't fill this out for them? 

Thanks for the help!!