| In the UK and Australia we call all-purpose flour 'self raising flour', which gives you an indication of its effect - if you use plain flour, there's no raising agent included and for example your cakes won't rise during cooking. If you run out of 'self raising flour' and want to bake a cake, you need to add what we call 'baking powder' ( a mixture of sodium bicarbonate and sodium acid pyrophospate mixed with rice flour, according to the packet). I assume this is what's added to all-purpose flour anyway.
Of course you can use plain flour for everything that doesn't need to rise - eg crepes. | 
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