I recently received a below letter from home office.

Has anybody seen this letter before and any idea why they sending this letter.
I have sent them treaty right documents e.g. payslips for my wife to home office but i didn't send my passport and explained them that i need to travel and also sent them my ticket as a evidence.
I am waiting for their reply but there is no reply yet...Sent everything about 3 weeks ago.
Please advise.
thanks