This is a useful technique to make the site appear to be faster than it really is. You specify a low resolution version of the image that can be contained in a much smaller file. This is then loaded first by the browser while it downloads the larger image.
Although this property is read/write, changing it does not force the image to be reloaded again.
The DOM level 1 specification refers to this as the lowSrc property. Note the capitalization.
For this to work as intended, you must specify the lowsrc (lowSrc) property value prior to the src property value otherwise the high quality image will be fetched first.