Why do downloaded applications arrive as .dmg files When downloading most applications from websites they are usually packaged into dmg files.

Is there any specific reason why people distribute apps in dmg files instead of zip files? 

If so why do some apps still packaged as .zip?
.dmg files are self-checking, unlike .zip files &amp; so are less likely to error on opening. 

dmg can also execute &amp; mount to the desktop as a disk, which would need 3rd party software to do the same for a zip. 

They can also contain sub-folders, placed icons, background images for logos &amp; instructions etc