It is extremely important that you meet the FAFSA deadline. If not, you will not be eligible for financial aid for the upcoming year. In fact, it is very important that you complete it as early as possible. Some grants, loans and scholarships are awarded early in the cycle.
Submitting your FAFSA early enough is important because it
A) allows for processing and any updated or errors to come through the system with time to fix them
B) puts you in the running for some of the grants and scholarships that are only given by universities and colleges until they run out
Schools can't give away funding that they no longer have. The early bird will sometimes get the worm in this case. If you wait too long, you might only be left with whatever is at the bottom of the barrel or just loans.