Why does Android OS need custom ROMs for OS upgrades?
Your phone is from 2013 - quite old to be expecting software updates. Hence it's quite unlikely that you would get a major Android version update from LG (maybe security updates if you're lucky). Custom ROMs are maintained by their respective devs giving a bit of extra life for your device. 

As for the third question, the answer here is very compehensive - Bootloader/BIOS, flashing ROM and correlated risks. Why Android devices are more brickable than PCs?