Apple switched iPhones to HEIC by default in 2017 with iOS 11. The format is genuinely excellent. It compresses photos roughly twice as efficiently as JPEG at the same visual quality, which is why a 12-megapixel iPhone photo might be 3MB as HEIC but would be 6MB or more as JPEG. The problem is that HEIC was developed by the MPEG group and requires a licensing agreement to decode, which means most non-Apple software simply doesn't support it. Windows doesn't without a paid codec. Most web apps don't. Most upload portals don't.
WebP has no such licensing issue. Google developed it and released it as completely open and royalty-free in 2010. Every major browser added support within a few years, and today WebP is accepted by essentially every modern web platform. For iPhone photos destined for a website, social media, or any web application, converting from HEIC to WebP gives you a file that works everywhere while keeping the file size small.
WebP vs JPG for your iPhone photos
Both formats are widely supported and both are good choices for converting HEIC. The difference comes down to what you're doing with the file. If you're uploading to a government portal, a job application system, or any form that was built more than a few years ago, convert to JPEG. Those systems were built assuming JPEG and will sometimes silently reject anything else.
If you're uploading to a website, a modern CMS, a social media platform, or sending to someone who will use the photo on the web, WebP is the better choice. The files are 25 to 34% smaller than equivalent JPEGs, which matters when you're uploading dozens of product photos or blog images and paying for CDN bandwidth, or when you care about page load speed.
What happens to your photo's quality during conversion
HEIC uses a compression method based on the HEVC video codec. WebP uses a completely different algorithm derived from the VP8 video codec. They're not directly compatible, which means converting isn't a lossless transfer. The conversion decodes the HEIC pixels and re-encodes them as WebP. At quality 80 to 85, the visual result is essentially indistinguishable from the original for photographs. You'd need to do a side-by-side comparison at full zoom to see any difference, and for most use cases you'd be looking at something that doesn't matter in practice. The default quality of 82 in this tool is set specifically to give you that balance.