These will be if you think the following vehicle is unable to see the obstruction for some reason, or if the obstruction is in a particularly dangerous place, like on double yellow lines or a clear way, and perhaps on a bend, or at any time you think the following vehicle might not expect you to be pulling out.
If the following car seems to be preparing to overtake you but has not actually committed it yet, then a signal is necessary because the other driver is obviously not concentrating and needs to be alerted to the danger ahead. If the following car is committed to an overtake, then it is better to yield by slowing sufficiently to let it by without yourself having to actually stop for the obstruction.
This is very rare though, and you mustn’t do this except when really necessary, like to avoid an accident due to the other driver’s foolishness. Some candidates do it all the time, for anything following, and they go down for lack of progress.
Bear in mind too, in all this, that if safe you should be doing the legal limit or very close to it, and technically, therefore, no-one should be trying to pass you. Yes, I know, don’t laugh!! Hardly anyone keeps to the limits these days, but it means the extra speed of an overtaking vehicle is illegal, and they have therefore themselves created a situation that is not without danger. Just make sure you do the right thing, as above, and let the others look after themselves.