Not having to redo the entire chain of image operations from scratch if you mess up one step. The ability to reuse the chain of operations on a different image. The ability to tune / tweak the settings of each step without having to repeat the entire chain manually from the start. The ability to edit the chain of operations if you don't like the result, without having to re-execute the chain manually.

And, lastly, node-based systems can be incredibly fun, if done right.