I'd presume it's simply to assist with establishing if X route and Y flight is primarily used for leisure or business travel. Whilst many routes in-of-themselves can be defined as one or the other I imagiei knowing either way will help BA track current trends and refine if on route X flights 1,3,7 and 9 are business heavy, the rest vice versa and any seasonal/ annual changes to that trend.
Obviously marketing will be targeted quite differently and it could affect charges if the route is highly competitive.
Totally agree with whomever posted just before me( on phone can't see) in that a GDPR request, whilst possibly overkill, would get you the full answer .