Huh?
To verify that you were actually off sick 
Of course.. any long term sick they are supposed to do a home vist to see how you are.
They have a policy that also says Im supposed to phone in and give regular updates on how I am
If I put a sick note in for 4 weeks Im not phoning them again unless Im going back to work within those 4 weeks
Companies have policies, policies are not laws, I tend to ignore most stuff.