OK, last try here.
Many people carry a potty for a few weeks when their toddler is first toilet training in case they need an unscheduled wee/poo and toilets aren't quickly available.
Most of these people will, if possible, use nearby toilets if available. But quite often they aren't (esp in residential areas where most of us tend to live).
Most people encourage their children to use the toilet before going out.
Most small children are not 100% reliable, especially in the first few weeks out of nappies.
Then (and this is the good bit) after a few weeks the children get the hang of the whole thing and the potty is no longer necessary (unless you are planning a couple of hours at a park with no toilets).
The upshot of all this is that, very occasionally, one is exposed to the sight of a young child on the potty (or being held out over a gutter).
Some people think this is a problem, some don't, and some have never seen this hence it doesn't exist except on Mumsnet (a bit like 3rd world poverty and Blue Whales I guess).
And now I'm off to the loo. 