The apt command also merges functionality from apt-get and apt-cache.
With apt you don't have to use two separate tools to search for packages to install them. That's probably at least a moderate usability improvement for many people since you don't have to figure out whether you need to use apt-get or apt-cache for some particular command.
The primary difference between apt and apt-get is apt has better dependency resolution and apt cleans up after itself when you do upgrades (it uninstalls packages that are no longer used).