Form I-131 is used to apply for permission to travel outside of the United States. It is used in multiple circumstances. One example is when a person who applies for a green card via the Adjustment of Status process. When applying for a green card, the person may also apply for travel permission. Adjustment of Status applicants who exit the U.S. without approved travel permission abandon their green card applications and their cases will be denied.
There are many applicants that should not travel outside of the U.S. even if USCIS approves travel permission to them. Those include foreign nationals who may be barred from returning the U.S. because of prior periods of unlawful presence in the U.S. or for other reasons. Because of the many rules involved regarding travel permission as it relates to your case it is important to work with a competent immigration attorney.