The short answer is that you need them if you have a green card and are going to be out of the US for more than one year.   You normally can’t return to the US after a trip of more than one year if you have a green card and you don’t have a Reentry permit.   A Reentry Permit will let you stay out two years (sometimes a little more) — as long as you don’t do something during that time that strongly evidences that you have abandoned your residence in the US.

Under certain circumstances, they will give you less than two years time for a Reentry Permit.   The main time is if you have a two-year conditional residence card — your Reentry Permit will expire when your two-year card expires.   The other main time is if you have been out of the US for more than four of the last five years — in that case, you can only get a Reentry Permit for one year.

If you can only get a Reentry Permit for one year, then what is the purpose of a Reentry Permit?  After all, if you can only stay out one year, you can always reenter with a green card, right?    The answer to this is, “Who knows?”   In my experience, no one really knows the point of a Reentry Permit — expecially if they are border guards — who don’t seem to have any clue.

The main point of a Reentry Permit is that it is evidence of your intent to remain a resident of the US, and, therefore, they should not hold it against you if you have been out of the US a lot.   That is why even a one-year Reentry Permit should be helpful on the border or in an airport.   Although the green card is enough to reenter the US if you are returning from a trip of less than one year, if the Border Patrol/Custom officer sees that you have been out of the US for most of the last four or five or six years, they may say you no longer have the intent to be a resident of the US, and could take away your green card.   A Reentry Permit is supposed to protect you against that.   But to be honest, I have seen returning residents, who had just recently obtained their green cards, and who had a Reentry Permit, and had been out of the country for much less than a year, be given trouble by a Customs officer at an airport, and the Customs Officer didn’t even want to see the Reentry Permit.

So, in short, if you do a lot of traveling, even if you never are out of the country for more than a year, a Reentry Permit should be helpful, and I do recommend it.   However, it appears that the Border Patrol/Custom officers tend to only treat it as a permit to stay out of the country for more than one year, and disregard its other, main purpose — that it is meant to evidence that you have the intent to keep your residence.   So, it is good to have, but is it worth paying the money if you are not going to be out of the country for more than one year?   I don’t know!