If you have a green card or permanent resident status in the US, you will want to hold onto that status. Your conditional resident green card or permanent green card is an important asset that lets you live and work in the US. While a permanent green card is permanent, you can lose your status in some cases if you stay outside of the country for an extended period of time. As well, remaining outside of the country for an extended period of time can affect your residency requirement for US citizenship, if you decide to pursue naturalization.
If you may be out of the country for an extended period of time, you will want to ensure that you apply for Form I-131 (Re-entry Permit) before you travel in order to protect your green card status. It is especially important to apply for Form I-131 (Re-entry Permit) if you are a conditional resident or have immigration petitions pending. If you leave the country, even for a shorter period of time, with applications pending or while a conditional resident, you could be seen as abandoning your petition. Filing Form I-131 (Re-entry Permit) shows that you are not abandoning your petition and filing the form also ensures that you will retain your status when you re-enter the US.
You must file Form I-131 (Re-entry Permit) and receive an acceptance to your application before you leave the US if you are a conditional resident or have petitions pending. If you do not, you may lose your status, even if you only travel to Canada, or just beyond the border to Mexico. In some cases, some H, L, V, K-3, and K-4 visa holders may not need to file Form I-131 (Re-entry Permit). As well, some refugees and asylum seekers who are seeking adjustment of status and have lived in the US for at least one year do not need Form I-131 (Re-entry Permit). However, since the consequences of not having this travel document are so dire for your chances of becoming a permanent resident and getting a green card, you will want to carefully make sure well before you travel whether or not you need a re-entry permit. If you do need one, do not leave the US even temporarily without this document in place.
Keep in mind that while Form I-131 (Re-entry Permit) is vital in many cases for conditional residents and some green card holders, it does not guarantee a loss of status. You are not guaranteed re-entry into the US if you have this travel document. You are also not guaranteed that your pending petition to become a permanent resident will be accepted if you have Form I-131 (Re-entry Permit). Form I-131 (Re-entry Permit) is simply a way to ensure that you do not automatically lose status when you leave the US.