Gazeem
At any particular moment in time the controller must be able to prove separation from other aircraft. If you are in level flight on airways and an aircraft is coming the other way [for example] it may well be that vertical separation is assured; however if you or the other aircraft are climbing/descending through each others level the controller may be using Radar separation, and in order to demonstrate the safety you may be instructed to fly an assigned heading.....sometimes called "radar heading" although the "radar" part is not strictly necessary but it is in common usage as a recognised and unambiguous phrase.......!
If the routes or tracks of the respective aircraft are sufficiently separated from each other a "heading" may not be required, but in the very dense UK environment the tightly packed airspace means that positive control in the form of heading assignment is the norm......basically it means that ATC can be absolutely certain that you will not deviate from your heading and can therefore climb/descend traffic through your level using radar separation standards such as 5 miles.
The point you raise is simple....ATC need to "lock" you on a heading for radar separation purposes.....but there may be no need to take you off your flight-plan track.....you can be cleared direct to somewhere and then "locked on". What we can never do is to think "well he probably won't turn and so I'll take a gamble and climb/descend through with the other one, who likewise probably won't turn"
We don't do "gamble".