iPhones save photos as HEIC by default since iOS 11: efficient on-device, but often unreadable when you try to open it on Windows, in an older Android app, or on a website that doesn't recognize the format. Converting to JPG fixes that immediately, since virtually every device, app, and platform accepts JPG. Because the conversion happens entirely in your browser, your photos never leave your device. If you have a whole camera roll to convert, DuckConvert's dedicated HEIC to JPG Batch tool handles many files at once.
A photo format almost nothing can open
HEIC wraps a still image encoded with HEVC, the codec built for 4K video, inside a HEIF container. Apple switched the iPhone camera to it because it halves the storage a photo library needs at comparable quality. The problem is not technical merit but licensing: HEVC decoding carries patent obligations, so support has to be bought rather than assumed.
That is the reason Windows points you at a paid codec extension, why plenty of web upload forms reject the file, and why older photo editors and print kiosks show nothing at all. Converting to JPEG trades away the storage advantage for a file that every piece of software written in the past thirty years can read.
The colour depth you give up
An iPhone can capture HEIC with 10 bits per colour channel plus HDR gain information, which is what lets a bright sky hold detail instead of clipping to white. JPEG is fixed at 8 bits and has no HDR concept, so the conversion tone-maps that wider range down into the narrower one.
On most photos this is invisible. On a high-contrast shot, a sunset or a backlit portrait, the converted version can look slightly flatter in the brightest and darkest areas than it did on the phone screen. Orientation and the EXIF block, including the timestamp and any GPS coordinates, do carry across to the JPEG, so if the location data is the reason you are converting, run the result through the EXIF stripper afterwards.