They say the best camera is the one you have with you, and your phone is usually with you. In any case, some professional photographers actually prefer shooting on their phone even for planned, high-profile shoots—perhaps they like its convenience, or that its unassuming nature puts subjects at ease. Or perhaps they find it creatively freeing to be burdened down by only minimal gear.
For example: https://www.gadgetmatch.com/time-covers-shot-iphones/