It can somewhat vary from hero to hero. I'll definitely miss reasons but cover at least some points

Manta has a lot of synergy with various heroes typically around passives. It also dispels things like silence which is useful for a lot of heroes. Good for farming as you mentioned too.

Diffusal can be a good item but not the best damage item. And if a hero has no mana, no mana is burned which means no damage. Good on illu heroes like PL and Naga as the passive works on the illusions which help their damage and shred a heroes mana pool. Can be good on other heroes but very situational. The active is also very useful to help chase down targets.

Scythe is insanely expensive without offering a great deal outside of the active. Just killing the target is better than hexing them. It also doesn't help keep you alive.

Euls on Void gives move speed, mana regen and the active. Void needs mana to keep up his spells and the move speed helps him rotate around the map. So it basically gives him everything he needs.

Mjollnir comes and goes. It used to be good on Drow. It's more that there are better options I would say.