You as a lawful permanent resident or a conditional permanent resident do need a Re Entry Permit to travel outside the USA in case you plan to be abroad for more than 1 year but less than 2 years. It allows re-entering the United States upon returning from abroad, without having to obtain a returning resident visa.
If you stay outside the country for more than 1 year without a Re Entry Permit, you will most likely be denied reentering into the USA on the ground that you abandoned your permanent resident status.
Meanwhile, you do not need a Re-entry Permit if you will be outside the USA for less than 1 year. In this case, you may use a Permanent Resident Card (Form I-551) as your travel document.
Please note that permanent residents must maintain ties to the United States anyway and the travels must be temporary in nature, as even with a Reentry Permit you must meet the continuous residence requirement in order to pursue naturalization.