How this converter works
The browser decodes each PNG, redraws it on a canvas, and exports a JPG at the quality you pick with the slider. Since JPG has no alpha channel, transparent areas are filled with white. Nothing is uploaded; a whole folder converts locally in seconds.
The size argument, in real numbers
This conversion exists for one reason: weight. A photographic PNG is routinely five to ten times the size of a visually identical JPG at 90 percent quality. That difference decides whether an email attachment goes through, a form's size limit accepts the file, or a page full of images loads fast. The slider is the whole tradeoff: 90 and up is visually lossless for photos, 75 to 85 is fine for casual sharing, lower starts to show.
When not to do it
Two cases where JPG is the wrong target. Transparency: if the PNG is a logo or cutout on a transparent background, JPG replaces that with solid white, which is usually not what you want. And sharp-edged graphics: screenshots, diagrams and text develop faint halos under JPG compression. Photos convert beautifully; interface pixels and logos are the ones to leave as PNG, or push through the opposite direction when a form insists.