Master Lock Key Safe in Holyhead

As a reformed man of ill repute when younger, this is not true. These can be easily knocked off a wall with one swift tap of a brick/hammer to be attacked elsewhere (DIYers never fit these securely)

As for gaining entry without a key, it's usually noisy and with modern double glazing and composite doors virtually impossible to get in 'without' a key (watch an episode of Police Interceptors to give you an idea of how strong composite and UPVC doors and windows are)
Modern multi lock doors (don't forget to update to 3 star EURO lock protection) and windows take a lot of force, so by conveniently leaving a key in an unsafe box have created an easy way to get in, then out 'and' they can lock the door behind them leaving no trace (or to get back in to finish clearing it out)

Personally I would leave a key with a neighbour or whoever waters the plants but what would I know.

Just like their padlocks. They're so bad that a Youtuber took a bunch of these locks to the US manufacturer and videod himself opening them in the car park.

Some key cutting machines will do you 2 keys for not much more so I really can't understand why these exist (maybe emergency entry for the massively paranoid?)