I built this PC and my Gigabyte motherboard came with some unwanted
bloatware such as a self-updating bios program and a driver that
allegedly speeds up the charging of MP3 players and such.
While I can attest to the fact that bloatware is bloatware, the driver/software does allow quicker charging of USB devices (e.g. cellphones, MP3 players). This isn't a part of the standard USB-spec, so this is why you need additional drivers/software to use it. It's built into the hardware, yes, but you do need a special driver or program to use it.
All of these things in the control panel that I have uninstalled were
listed as published by Gigabyte. One in particular that seemingly also
came preinstalled was this "NEC Electronics USB 3.0 Host Controller
Driver". What exactly is it? Do I need it?
It depends where it shows up. If you're talking in Add/Remove Programs, then most likely not. If it's in the Device Manager, however, then it all depends.
Your motherboard has both a USB 2.0 and 3.0 controller. Disabling that driver will effectively disable the USB 3.0 support, and with it, any connectivity through those ports. Even USB 2.0 devices won't work on those ports (they will work on the other USB 2.0 ones). Just incase you didn't know, the blue USB ports are USB 3, and the black ones are USB 2.
If you have no use for USB 3.0 and don't see a use for it in the future (it can always be re-enabled), you can disable it. If you do use the USB 3 ports, and/or have USB 3 devices, then you may wish to keep the driver/software enabled.