Yes, you get EAD several months in advance to your GC. With EAD you can apply for Social Security Number and get it. With SSN, you can claim your parents as dependent on your tax, if you want. With EAD and SSN they can start showing income $3600/yr minimum , i guess, to qualify for 4 points/year for their Social security benefits later on.
They get good looking EAD card with their photos that makes them and you feel good.
Can they live without EAD , sure, they can , GC will be there within 4 months (from EAD) any way.