Roof rails invite loading, and the weights involved look small against a vehicle's payload. Their position is what makes them consequential rather than their mass.
Height multiplies the effect of the weight
A vehicle's resistance to rolling depends on how high its combined centre of gravity sits relative to its track width.
Weight placed on the roof is the furthest possible point from the ground, so a modest load raises the centre of gravity more than a much larger load in the boot.
This is why manufacturers specify a roof load limit far below the vehicle's payload, and why the limit is expressed as a maximum rather than a guideline.
Handling changes before anything feels wrong
Body roll increases and the vehicle takes longer to settle after a direction change, because the mass being controlled is now acting through a longer lever.
Emergency avoidance is where this matters most, since the second movement of a swerve arrives while the body is still returning from the first.
Electronic stability systems intervene earlier as a result, and their calibration assumes a normal load distribution rather than a loaded roof.
Braking follows the same geometry. Weight carried high transfers more aggressively forward under heavy braking, unloading the rear axle sooner and bringing the anti-lock system into play at lower deceleration than the driver expects, particularly on a wet surface.
The dynamic load exceeds the static one
The quoted roof limit applies to the vehicle in motion, and every bump multiplies the effective weight acting on the mounting points.
A load that sits comfortably when parked can generate several times its own weight in force over a sharp compression at speed.
This is also why the limit applies to the rack and its contents together, and why the rack's own weight has to be counted against it.
Aerodynamic cost is larger than expected
A roof rack sits in the fastest-moving air over the vehicle and disrupts flow that designers spent considerable effort smoothing.
Empty crossbars alone raise consumption measurably at motorway speed, and a box or bicycles raise it substantially more.
For electric vehicles the effect is more visible because range is displayed continuously, but the underlying penalty applies equally to combustion vehicles.
Crosswind sensitivity is the least anticipated change
A tall load presents a large side area high on the vehicle, which increases the turning moment a gust applies.
Exposed bridges, motorway cuttings and passing high-sided vehicles produce the sharpest inputs, and the vehicle responds more than the driver expects.
Removing racks when not in use addresses all of these at once, which is why most manufacturers recommend it rather than treating a rack as permanent equipment.