Good question this crossed my mind too. You can say it is for visa purposes that you wish to travel abroad and the visa requirements include this letter. This is technically truthful.
My letter said
To Whom it May Concern:
The purpose of this letter is to verify that [Full Name] is currently an active, full time employee of [Company Name]. [First Name] is employed with us as a [Job Title] and has been employed with us since [Job Start Date]. He is currently receiving an annual Salary of [Salary Amount]
Should you have any questions, please feel free to contact me at [Company Phone Number]
Best Regards