HEIC is the format iPhones and iPads save photos in by default. It is efficient, but plenty of websites, older Windows software and upload forms simply refuse to open it. Converting to JPG produces a file essentially everything can read.
Turn an iPhone HEIC/HEIF photo into a JPG or PNG anything can open — entirely in your browser.
Live Photos are stored as a still image plus a short video. Only the still frame converts here — the motion component is not part of the resulting JPG.