Main question- There is almost no significance of orbs in paranormal photography. Orbs have many causes and usually cannot be used as evidence towards anything paranormal. Actual "ghost" orbs and regular ol' dust (or whatever) orbs are indistinguishable (imo). However, there is the rare occasion where an orb will coincide with some other piece of paranormal evidence. Depending on what side of the fence your on, this can be proof or this too can be waived off.
Question 2- People do see orbs. It is just another form of energy manifestation.
And no, all orbs seen with the naked eye are not ball lightning. I have heard this many times.