Well, faster booting, faster network configuration, native support for large drives, better design to handle higher-end video chipsets, and yes, more security (if you want it - can always disable that). Booting a system from a BIOS requires the host OS (whichever it is) to do a LOT of device handling, memory manipulation, and other things just to boot properly and make sure everything works and is mapped out properly. Also, a UEFI system abstracts most of the hardware from the OS, so a driver or OS developer no longer needs to know all the ins and outs of how certain things work at a firmware level, and can target code at the device level. This makes OS code and driver code that speak directly to hardware potentially smaller and more efficient, and lets the UEFI OS (yes, it is an OS) underneath the booted OS handle hardware and driver interaction at that level, making things faster.
BIOS isn't bad, per se, but UEFI is certainly better from a system design and performance standpoint, not to mention supporting the future of devices - BIOS is limited to 1MB for it's ROM, meaning it does sort of hold hardware design back. Having a UEFI (64bit) boot environment allows such faster booting and security to occur, and gives more headroom for hardware designs that will be coming forward in the future to move off of designs that currently rely on BIOS extensions to "make things work" in BIOS land, where they could be rewritten to use the native UEFI OS capabilities instead.
Once your OS is up, other than some performance gains, you probably won't notice as an end user. However, if you're a developer that deals with hardware, your life gets a bit better and potentially easier. Also, if you're supporting things in IT, you now have the ability to be more secure and functional (think Intel V-Pro as well) versus BIOS systems.
Microsoft initially switched to supporting UEFI boot in Vista (x64) for these reasons, and it's why Win7 and Win8 platforms (including Server 2008, 2008R2, and Server 2012) also have this support. Getting native support for 2TB+ drives and removing the 4 primary partition limits on disks is nice too, but it's hardly the primary reason. UEFI systems also allow antivirus and other such security pieces to be loaded down in the UEFI OS level (which is coming from Intel, very soon) that OS security subsystems can tie into to provide security in that way as well.