In the past, Destiny 2 doesn't have the cross-platform matchmaking, so users are not allowed to play with their friends on PS4 (or other platforms). The Destiny 2 Cross Save Feature Is Amazing This page introduces this feature and shows you how to enable it.
Destiny 2 designers finally make this come true the Destiny 2 cross save feature is added to allow users to sign in with a Destiny 2 player account from different machines.
Many people want to save their progression on different devices so that they can continue to play the games on multiple platforms.