Your form 470 preserves your residency in the US even though you travel outside the US. It makes it possible for you to apply for citizenship based on your residence in the US (certain amount of residence time is required for citizenship).

The reason you also need the 327 (reentry permit) is that a 470 does not give permission to reenter. Therefore, the 2 forms are required, one to preserve your residency time for your future citizenship application, and the other one to get you back into the US after your time abroad.

If the government gave you residency credit for the 2 years you were out of the US (which often is restricted for only certain types of situations, such as religious missionaries or people working for US companies abroad) then you would have 3 years left to reside in the US before fulfilling the 5 year requirement needed to apply for citizenship.

However, if you didn't get permission on your 470 application to count the 2 years you were outside the US, then you still would have about 5 years to reside in the US before applying for citizenship.

I -470 is to avoid the break of your continuous residence period in the US. Not everybody can file this form. As mentioned above, this form is used only to preserve the residency and you cannot re-enter using this.

A total of 5 full years is a must while satisfying the continuous residence criteria. If your I-470 is taken into account, then you need to wait for another 3 years before you can file in your citizenship application. Otherwise you can file it only after 5 years. You can check out the link to get a clear idea

http://www.uscitizenship.info/blog/the-us-citizens hip-application