In the UK en-route we will ask so that we can tell the destination airfield if there is any problem, it's usually something they will ask anyway.
"Operational Reasons" is enough for us and it's just to ascertain you don't need any special treatment. If it's in another country then we'll just alter your flight plan so any other agencies on your routes, new or old, are expecting you, or not, and to make sure you're not subject to any flow restrictions anywhere