Ordinary digital images in RGB format only contain up to 16.7 milion colours (8-bit per colour channel and one 8-bit alpha channel, making it a total of 32-bit). A HDRI (High-Dynamic Range Image) can contain a helluva lot more. By taking a picture in several different exposures and combining them into one HDRI, you have an image with colour data way more than 24-bit. You have 16-bit per colour channel, making it a 48-bit image. If you would add an alpha channel, it would be a 64-bit image. HDRI is used to light environments (quite an easy way to make it look somewhat realistic if you already have a HDRI) and for reflection. Real-life speculars are in fact reflections of the light source. To calculate this in 3d takes some time. That's why we use specular shading (blinn, phong and so on). An HDRI reflection, however, can create more realistic looking speculars, since they are based on the HDRI's brightest spots, and not point-lights (ordinary omni, spot-light and directional)