Power plug orientation and usability
In some applications, polarity matters because there is typically a policy of having switches and fuses all in the "hot" path (from the fusebox right through to appliance internals).



The holes are for locking the plug to certain types of receptacle (maybe)

The world's smartest man says "Engineers have been sticking holes in the prongs of plugs for so long that they have forgotten why they started."



You leftpondian householders seem to have a baffling array of outlets to understand.


from Z-tronix


  The National Electrical Manufacturers Association (NEMA, USA) has set standards for configuration that differ depending on current rating (15, 20, 30, 50 or 60 A) and electrical potential (120, 208, 240, 277, 480 or 600 V)*. This has resulted in more than 35 different straight blade configurations (various grounding systems included). 


From Museum of plugs and sockets


  The system rules out accidental intermating of devices and related risks.


If you look at the DIY SE site you'll find several questions where leftpondians are having trouble changing plugs on appliances to fit their outlets, or rewiring outlets to suit new appliances. 



Luckily for rightpondians, their homes have exactly one type of outlet (two if you have a shaver outlet in bathroom) so life is a lot simpler. Anything over 3.1 kW gets hardwired by an electrician.