http://www.wegreened.com/I-140-and-I-485-concurrent-filing

What are the primary advantages of concurrent filing?

There are several advantages of concurrent filing of I-140 and I-485, here are the primary ones:

File I-485 earlier: You, your spouse, and your unmarried children under 21 can file your I-485 applications (i.e., green card applications) at the same time your employer or you file an I-140 Immigrant Petition for Alien Worker. (Normally, the employer files the I-140, but you can self-file your I-140 under certain categories, such as Aliens of Extraordinary Ability and National Interest Waiver.) Also, the concurrent filing rule allows you, your spouse, and children to file your I-485s while a previously filed I-140 is still pending.

Obtain work authorization(EAD) and travel documents(AP): When you file your (and your family members') I-485, you can also file for Employment Authorization Documents (EADs) and travel documents(AP). The availability of an EAD is a major benefit for spouses who could not otherwise work legally (i.e., for spouses who do not already have an H-1B, O-1, J-1, etc.). Advance Parole can be a major benefit for those who could not otherwise travel (e.g., are out of status) or who do not want to risk getting stuck overseas because of security checks or other problems at a U.S. consulate while applying for a new H-1B visa.

Might be able to leave job earlier: Having an EAD for the principal beneficiary does not necessarily mean you can leave your current job, but in conjunction with the portability provision, the beneficiary of a pending I-485 can leave his or her current employer and take a new job in a same or similar occupational classification after the I-485 has been pending for at least 180 days. Please note, however, that many issues related to portability remain unresolved, so you should be very careful before you leave your current position. If you leave too early, your (and your family members') I-485 (and everything else) may end up being denied. On the other hand, in some special cases (e.g., Aliens of Extraordinary Ability or National Interest Waiver), you might be able to leave your employer even before the I-485 is filed. Job-changing issues are complicated, however, so it is critical to get good legal advice before leaving your job any time before you get your green card.