Why the right matters more than it sounds
Flats share things. A pipe serving your flat runs through someone else's; a repair to your ceiling is work on their floor; access to a roof is through a stairwell you do not demise.
A right of entry is the lease's answer to all of that. Without it you are dependent on the goodwill of a neighbour and the cooperation of a landlord at exactly the moment — a leak, a failed roof — when goodwill is in shortest supply.