Well, yeah that is obvious.
Tho concept was that the Neb-B was created to do Convoy escort, and bring alnog a few fighters to combat the Rebel tactic of Hit and runs.
Typicall rebel attack on shipping lanes would either find very little escort, that did not have quick response for Reinforcments. Or they found a Star Destroyer. The Empire wanted to use their ISDs for other things then escorting convys (i think the captains wanted action)
The Neb-B was as a Anti-starfighter platform that was able to bring SOME fighter escorts with the convoys. Most captial ship have convergin Fire zones (we call them "money lanes") it jsut isn't as apparnt as the Neb-Bs fire zone.
Now, this plan probly would have worked but for 2 reasons.
1. 1 Neb-B is not enuf (B-wing was designed to take out the Neb-B by itself)
2. Defecting captains (why do you think the rebels had an imperial ship)
Like hte Lancer class frigate, it was too little, and after the devoplemnt of the B-wing as the new rebel Stike craft, too late