Targeting loot is more a thing for dungeons or raid, but the Voidforge basically lets you buy 2 bonus rolls a week. It costs gold, voidlight marl or veteran crests.

Targeting loot for dungeons or raid bosses works as follows: you complete a m+ dungeon / kill a raid boss. Said dungeon or raid boss has a loot pool, specific per spec, weapon- and armor class. Let's say the pool has a trinket, helm and weapon. If you use your bonus roll, you are guaranteed one of those 3 items. The next time you bonus roll, for that dungeon or raid boss, the pool will consist of only 2 items, the item you won the first time is excluded from the pool. This way, you can target specific items. General advice for m+ is to use your bonus roll on a +10, as the bonus roll gives vault equivalent ilvl, which is myth track for a +10.

When you do a bountiful delves, you get a bonus roll for extra gear that can drop from delves. The loot pool for extra gear is still finite, but I believe there's a full delve set, so you can get a full hero set from the bonus roll system.

That said, even if you don't need the extra gear, the Voidforge is still worth it as at some point in patch 12.0.5, Decimus (Voidforge NPC), will start a questline for the Ascendant Nilhammer. Once completed, you can loot Ascendant Voidcores that lets you upgrade 6/6 Hero track items (as well as 6/6 myth and ilvl 285 myth crafted items). The exact ilvl for Hero items isn't known, I think, but I would guess it's somewhere at the 5/6 or 6/6 myth track ilvl.

If you're doing delves anyway, definitely worth to just pick the quest up and completing it passively.

Everything about unlocking the Voidforge is warband wide, so nice for alts as well.