What he means by applying for ead is when you send you aos package including form I-765 to get ead while you wait for approval of green card. Some people don't apply because they just recently received one with daca. What he says let's just say you send your aos package and trump removes daca st the same time your ead would be invalid and if you have a job you would lose it but if you send form I-765 upon approval they send you a ead card based on green card petition so you would still work. Is free If you send with your aos package. Some people like me aren't working so I wouldn't send I-765. I hope I explained myself well