One big difference between Oline and Dline. Dlinemen are constantly rotated in to keep them fresh while starting Olinemen play nearly every snap for continuity.

So when you draft Olinemen when you already have starters you usually don't get much from them. That's why typically those guys are late round draftees and UDFAs.

OTOH, the Dline depth are often higher draft picks because they will get playing time even if there are no injuries.

But yeah, both are the heartbeat of the team.