When I lived in the UK, I thought the answer to that question was fairly straightforward. And though I might have had an inkling that it wasn't quite that simple scientifically, it didn't really matter to me.
However, where we live now, water boils at just over 96 degrees Celsius. We live at a height of about 1100m above sea level - and can see mountains from our house and neighbourhood! To put that into context, Snowdon is 1085m above sea level. So we live at a higher altitude than the highest mountain in England and Wales (though not quite as high as Ben Nevis in Scotland, which is 1345m). We look out onto a 'small' mountain from our flat and L and I have been talking about walking up it one day. But our 'small' mountain is about 1800m high. Whereas the huge mountain that we can see if we walk about ten minutes from our flat is just over 3900m high.
As far as I understand it, the simple explanation for why water boils at a lower temperature here is that as altitude increases, atmospheric pressure decreases. Lower pressure causes water to evaporate more quickly and so water boils at a lower temperature.
To be honest, relative to the rest of the world we don't live at a particularly high altitude, just relative to the UK!
Living at this altitude doesn't actually make any difference to our daily lives (although L did try suggesting once that the reason he ached more after a run was maybe due to the altitude, especially as he ran half way up our 'small' mountain. Though I'm pretty sure it had more to do with the fact that he hadn't been doing much running for a while).
However, it does impact cooking and baking. Water never reaches 100 degrees Celsius, so foods cooked in water (e.g. pasta and vegetables) can take a few minutes longer to cook. And according to what I've read, rising agents such as yeast, baking powder or bicarbonate of soda can create a larger rising effect while additional liquid might be needed in baking recipes to compensate for the faster evaporation.
I'm not sure I really notice the effects on cooking and baking that much, but it's nice to have a scientific excuse to fall back on if something doesn't turn out right: "oh, the recipe was written to be cooked at sea level so I probably need to adjust it a bit next time to take into account our altitude."
