As in PIR ones in the garden to deter thieves. Are they worth it? Or do they just help the thief out so that they can see what they’re doing?

they’re good for illuminating said theif so you can throw boiling tar over him from an upstairs window, That’ll learn em.

They are probably more useful than not. I use mine so people don’t trip when they’re walking up my stairs, and I guess they put theives off a bit, because when your breaking into a house, who wants to be all lit up by a 500 watt flood light?

If you can find one, get one with a linked wireless chime.
We have one in the carport and provided the sensor is set so that it ignores our cat it means that if ANYONE comes on our drive at night we hear a chime. This has been really useful and has meant that I have been able to know when prowlers have been in our garden.
Obviously your location and setting up the sensor to avoid false alarms are key in this, otherwise it would do your head in.

They’re a good deterrent. Don’t underestimate how unnerving it is to suddenly be illuminated with 500W; particularly when it means that you can’t see who is/may be looking at you.

They’re never going to stop a determined thief (I know of one house where burglars smashed the light before breaking in), but they don’t do any harm.