Some of them are tosh and are purely there to make it seem like there are loads and loads of bargains.
However, I was browsing yesterday and spotted a 'lightning deal' on a P&S camera (
Fuji S1) at about £200 - or about 50% off.
My instant thought was that it was either a 3 year old model and the '50% off' was from the original RRP, or that the RRP was never actually that high.
Anyway, using
CamelCamelCamel I could track the price changes on Amazon since it was launched (Jan 2014) and found it was selling at £370 until about September.
Had a look around the web and couldn't see it much below £300. So at £200 it seemed a bargain and I grabbed it. Two minutes later and the 'lightning deal' had finished and it was back to it's normal price since of about £280 - so even compared to that price it's still about a 3rd off.