One of the Cheddington guards actually prevented a suicide in quite dramatic circumstances not long ago, and this has happened elsewhere on other occasions as well, so it’s far from a useless scheme. The staff are also monitored more closely than they used to be, largely by Network Rail MOMs, and I’ve found them to be more visible as a result. Successful suicide interventions in general (across the national network) are on the increase, too.
There are different agencies as well. All the Watford DC stations appear to have somebody, even where they don’t have WCML Fast/Slow platforms, but these seem to be from a different contractor. The Wembley Central staff do have the WCML to deal with and they are usually particularly good, in my experience. I believe they’re meant to check that nobody is loitering in an untoward manner after each train has departed, which is broadly what happens, but they also help out with luggage and passenger enquiries and so on.