Those signs are also peppered well before immigration and are just for a very niche section of people who arrive on a special type of visa - Creative Worker for example - a short term work visa. Such people could normally use the eGates on their passport but are required to get a stamp and have the visa ‘activated’. The reminder at the baggage belt is in case they forget (some do and have to go back).

If they don’t then it can apparently throw up problems with their work. A lot are musicians and models, actors etc and apparently their passports are checked to make sure they have the requisite stamp. So it’s in their interests to do it.

Others might be coming in for an extended work visa, study visa and again it’s in their interests to get a stamp else it might lead to overstaying problems down the line.

So whilst you might not bother, the people it’s directed to are generally law abiding and keen to avoid issues and probably will bother.