It's largely a privacy thing and also adds a little to security.
uPVC doors don't generally have supplementary one-sided bolts like wooden front doors with a night-latch usually do. This can't-unlock-the- door-if-there's-a-key-turned-in-t'other-side functionality takes the place of bolting the door and means that a keyholder (e.g. your dirty stop-out wayward teenager), can't gain casual/un-noticed entry to the house (and invite their mates in for tea and toast at three in the morning).
Clearly, where the house belongs to an elderly relative who may be prone to ill health, or have poor mobility or maybe just doesn't hear the doorbell, then this functionality is not appropriate and you can get a version of the euro-cylinder with a different internal clutch mechanism allowing a key inserted from the outside to operate the lock irrespective of the status of the inside cylinder (in fact it is as if the inside lock was a thumb-turn).