Writing to ask why our company needs to write an invitation letter

Hi everyone,

I've received an email from a foreign college that wants our company to write an invitation letter for them to visit our company. However, my company has never written anything like that. We only receive emails and we answer we agree to the visit or not.
Please note that that college does not need a visa to enter our country.

My boss doesn't want to send that kind of letter. So, how can I ask them the reasons that we need to write them an invitation letter politely? Asking "why do we need to write the invitation letter" seems impolite And how can I refuse to write for them?

Could anyone give me an advice? My English isn't so good. I just want to answer the email politely.

Thank!