I'm new to Mac (well not entirely new I took an almost 20 year break), but this is the best way I can describe the 555x and 560x...
These are "business class GPUs" so they are designed to give you extra performance is applications that can leverage the GPU for performance boosts. For example, Photoshop or Apple Photos can leverage the GPU for a bit more specialized power. The iGPU has been getting better and better as this though so how wide the performance gain is would have to be reviewed in either benchmarks or your usage cases where possible.
If you have workloads that make great use of GPU acceleration like video and photo editing then you may see a value in jumping up. Otherwise, you're likely fine with the iGPU and the battery life savings it brings.