HEIC images is a lossless high compression format. It takes long time to display them and many applications doesn't support them.

JPEG image are compressed and very small. They are very common to many applications and web services.

When you take pictures with the camera of Apple Iphone the default setting format is HEIC. Many web services and application don't support HEIC at this time. So it will be handy to share your photos from the photo library right away as JPEG.