As a first level spell witchbolt really isn't very good, but I think it upcasts very, very well and I think it is worth having because of this. I can't think of a single spell that upcasts better.
If cast at 3rd level it will do 20ish on the first turn and another 20 or so every turn thereafter, automatically for 10 turns without using another spell slot. Further if you can crit with it you can make that 40 on the first turn and depending on your DM another 20 or 40 every turn thereafter. This is a lot of damage from one spell.
Cast at 5th level it can do 33 per turn, that is more damage than bigby's hand and you only need one attack roll hit to do it.
If cast at 3rd level it will do 20ish on the first turn and another 20 or so every turn thereafter, automatically for 10 turns without using another spell slot. Further if you can crit with it you can make that 40 on the first turn and depending on your DM another 20 or 40 every turn thereafter. This is a lot of damage from one spell.
Cast at 5th level it can do 33 per turn, that is more damage than bigby's hand and you only need one attack roll hit to do it.