images are much larger and therefore take more data/time to load on a page.
Depending on the type of image/industry/goal at hand you will want to use a
low-res image to save on load times and data consumption.
Consider a few of these examples:
(1) Your audience is predominantly on smart phones or mobile devices, thus
potentially using precious data to access your site, in this case high-res
images would be doing your audience a disservice as they would be paying to
load those large images.
(2) You have a photography based website offering free stock imagery or
backgrounds, in this case you may want to use high-res images for your
customers to access.