I take the travel potty out with us for emergency wees, not because I expect to use it but because I like to be prepared for any emergency.
Yes I take ds to the loo wherever they're available. Yes I encourage him to have a wee before we go anywhere and again upon arrival but what if he just doesn't need one then? I can't wee on demand so, by the same token, I don't expect ds to do so either.
Last week we'd walked to the post office and were still a fair distance from home when ds announced he needed a wee. I made him hold on as long as he could but after ten minutes he was desperate. I wasn't going to let him wet himself when there's an alternative.
To those who say he's not ready because he can't hold on, I disagree: he's been potty-trained a few months now and generally goes upwards of an hour or two before needing a wee, he doesn't need reminding and takes himself off to the toilet when he wants it and rarely has accidents; but sometimes the urge comes on strong and he needs to go - usually when we're in the middle of nowhere.
We've never got the potette out in a shop - on those occasions we just have to dump our basket and rush off to find the loo. Like I've said already, if it is too far away, out of service or we just can't get there quick enough with the pushchair (especially if dd is walking), then we use the travel potty.