According to wiki:
In PvE, a character's effective level and attributes are automatically reduced based on the enemy levels immediately around the player, and there can be numerous level scalings per area/zone. They retain access to all of their skills and equipment so the area is easier because of this, but should still be challenging to play.
The benefits of this are:
high level characters are prevented from killing enemies too easily and depriving low level characters of rewards;
low level content doesn't become obsolete once a character reaches a higher level.