ATC 'Maintain present heading' instruction

[Stephen]

Can someone explain me why sometimes ATC orders you to 'maintain heading' after being cleared to a certain waypoint - sometimes 1 hour flighttime or even more further down the route?

E.g. ATC clears you direct BLBLA and asks you to report your heading once established inbound BLBLA. Once established inbound BLBLA, you report your heading and ATC orders you to 'maintain that heading'

Has it something to do with different lateral separation requirements for aircraft flying 'own navigation' and for aircraft flying on an assigned heading?

I understand that ATC requires you to fly headings in during descent and approach in terminal areas, but a lot of times it also happens enroute at high levels.

Thanks for yor reply!
Stephen