Last year, we saw a major Mac malware attack that forced Apple to issue a security patch in Mac OS X. Though common for Windows and Android, malware is still a rarity on the Mac, but as it gains popularity, the malware risk also rises.
Gatekeeper is an attempt to reassure users they aren't inadvertantly downloading malware. By default, Mountain Lion will only let Mac App Store apps and apps from developers with an Apple-issued ID launch for the first time. In this way, Apple can make sure apps haven't been modified to include malware and developers aren't just malware operators in disguise. But users can still choose to install Mac apps from DVDs and downloads if they want.