You don't _need_ them, and IIRC _can't_ (in general) use them if you deal with non-perturbative effects, unless you do stuff like re-summing infinite series of

You don't _need_ them, and IIRC _can't_ (in general) use them if you deal with non-perturbative effects, unless you do stuff like re-summing infinite series of diagrams and that really makes interpreting the ultimately observable effect in terms of virtual particles interacting difficult.