The Purge has been a great PvE mechanic on the Exiled Lands map (when it worked properly). But Purges would sometimes spawn inside a base and that just messed up the immersion and fun.
So, what if the Purges on Siptah emanate from the Dark Tower? That’s where the Purge spawns in and then marches, in waves, to the intended player’s base. It may take the waves a few minutes to march to the player base (depending on where they have to path). And yes, an intended Purge may stop along the way to take out other players’ bases they encounter (“targets of opportunity”). By having the Purge start at the Dark Tower, there’s no “in-base” spawning and we still get to play “Tower Defense” to defend our bases (and see how good or bad we built it and its defenses).
For PvP, there are “human-initiated” Purges = other players to contend with (which comes with griefing and trolling and all kinds of other “gameplay”), but in PvE, we need an AI environmental factor.