People with tons of pictures. .jpg files are relatively smaller. That I know of, the tree goes like this: JPG PNG TIFF DDS RAW
Jpg is great. You can get some really small file sizes out of it which is great for websites, plus basically anything can open a jpg file.
Jut because it isn't always perfect doesn't make it bad. Obviously there are times when you will need to use other formats to get features that are unique to them. For example you are not likely to use a jpg for an animated image, because the format doesn't support it natively (unless you count motion jpg, but that is generally classed as more of a video format). So if you need to display a logo that needs to scale well a vector image format may be more suitible than a raster image format as well as potentially having smaller file sizes. But I wouldn't want to upload a game screenshot using a vector format.
For my renders I use exclusively png mostly because it just looks the best while keeping file sizes reasonable with lossless compressio (for example, a 2000x2000 px image at roughly 12MB).
If I am doing a render its png all the way for me. But for uploading to my website I use photoshops "export for web" option and dial the jpg compression as high as I can while still getting usable images because I like to try and keep my site as fast as possible for as many people as possible.
JPG is good for cameras (with sensors, so there are no razor sharp lines). But it can't handle text very well, so for graphics PNG should be used (hence the name portable network graphic).
