I tend to assume if someone is used to living with a disability he's probably also accustomed to sometimes, or in some situations, just asking for help in some situations. Other than that, I guess if I saw someone particularly struggling I might ask if he'd like help. He can either say, 'yes, thanks" or "no, thanks". Of course, I don't know if that's how someone who has lived with a disability would see it. Besides seeing someone particularly struggling, I guess I'd ask if someone need help in a particularly challenging situation (like a particularly dangerous street-crossing situation that's not "run-of-the-mill").
Other than that, I have at different times lived with leg injuries (in a case or two, long term), and there's really nothing much anyone else could do to make anything I do any easier.
I don't so much have this problem these days, but over the last few years there have been times when I was working on building up strength for walking (and later stairs). There was a time when even a little extra weight (like a mid-weight grocery bag) made those walks more of a work-out than was good; which meant that while an almost empty-handed walk up stairs seemed effortless and reasonably graceful, too much extra weight would turn more into a two-feet-on-a-step type of thing, and include a little being out of breath.
I'd assume that if a person had something like arthritis in knees or hips it could be a similar type of thing.
When this was "where I was once" with the leg injuries I really preferred to workout when I decided to - not when I had groceries or something else to bring up the driveway, walk, stairs, etc.; so having someone else just take most of the bags meant I could practice the task of walking from the car to the house without the additional matter of bags.
The one thing I still can't do is climb to get the top of the refrigerator; so I think with "mobility issues" climbing tasks may be the ones a person will have the most problems with.
I suppose one point is whether a person's disability is permanent, getting worse, or getting better. Some types of assistance (in a temporary, even if long-term, situation) can happen with a more willy-nilly, family/friends type of arrangement. A life-long or worsening disability probably requires something more stable than that.
Just some personal thoughts/opinion, of course....