Because we've just had a horrible scare.
I was sat in the living room about 45 minutes ago reading DS2 a story and giving DS3 a cuddle while DH was warming DS3's milk in the microwave takes about 1 minute in ours (no comments about using microwaves for it please - this isn't the thread - it could have been anything in there).
DH popped into the bathroom (downstairs one at the far end of the kitchen) just as he was coming out I looked up to see smoke pouring into the living room and kitchen full of white smoke.
DH opened the microwave and got the bottle out and switched it off and we opened all the doors and window to let the smoke out. It was a few seconds after that the smoke alarm finally went off. The alarm is tested regularly (actually we've got 2 - one for "smoke" and one for "flame" ie smokeless fire) - and often goes off even if you're frying something or if you leave the oven open for more than about 10 seconds when switched on. Infact it went off this afternoon so we know it works. But it still took several seconds for it to go off in this instance.
In the time it took for us to notice the smoke, and for the alarm to go off the smoke had alrady started going into the lounge/diner and into the hallway. The upstairs smoke alarm didn't go off as because of the shape of our hallway (low at the bottom and then gets "big" once you're half way up the stairs) the smoke (for the time being) was trapped in the downstairs part. A few seconds/minutes? later it probably wouldn't have gone off.
However it made me realise that if that had been at night we may not have heard the downstairs smoke alarm (kitchen is quite a "distance" from our bedroom and you can't really hear anything - DH can't even hear me shouting at the DS's if he's in bed and I'm in the kitchen). If that had happened and the doors had been open the hallway would almost certainly have been completely full of smoke by the time we were alerted to it......and that is our only escape route!
Thankfully this time nothing serious happened (apart from now having a duff microwave and DH a cold dinner with no way of reheating.....any ideas???) but it could have been so much worse.