covers it well. But I'm adding a bit more detail too.
Beat Up is used specifically for beating Blissey reliably. It outdamages EQ substantially, with a healthy team.
This is because in ADV Beat Up doesn't use the actual attack/defense stats of the Pokemon. It uses their BASE stats, which means for the one case of Dugtrio itself attacking Blissey the damage calc is done with 80 Attack for Dugtrio and 10 Defense for Blissey. That 8:1 ratio is a much more extreme ratio than if you use their actual final, invested stats, which is between 2 and 3 :1. Even factoring in that EQ is higher BP and STAB, a combined 6 hits from most any reasonable set of Pokemon will end up outdamaging EQ.
You don't even need to pair it with physical attackers, since many pokemon on a special offense-spam team (which has always been very dangerous, if you can just get around blissey) also have high attack, since many of them generally are high BST legendaries. Zapdos, Jirachi, Celebi, etc all have higher attack than Dugtrio does! It's also not remotely impossible to engineer this sort of scenario where you trap Blissey early, with your team healthy. You can easily lead Zapdos with Baton Pass. They lead Skarmory or Suicune, etc, and decide hard switch to Blissey.
For the record, also pairing with Tyranitar in particular does not have any benefit as Beat Up deals typeless damage prior to BW.
In terms of fitting Beat Up on Dugtrio, Rock Slide was never particularly important but was just the "best remaining" move it could have, as it's generally a bad move to lock into and is primarily used against threats you don't trap, while Aerial Ace is very specifically for Heracross (and Breloom) and HP Bug for Celebi. Beat Up is likely to be better for most teams. Even in the situations where you don't get the chance to trap Blissey with 5 or 6 healthy mons, the opportunity cost of dropping Rock Slide is reasonably low. You would only use it anyway to revenge like... Zapdos, Mence, and Moltres, and even then you'd have to be in a pretty bad position to need to do so, since locking into RS is usually bad anyway.