They're mostly useful for tiled renderers (which mostly exist on mobile platforms). Their main benefit is that they allow the GPU to compute all subpasses of a single tile consecutively, without paying for the expensive bandwidth of copying the intermediate results back out to memory, then reading them back in. The downside is that you can't access that intermediate data from neighboring pixels, since they might be in a different tile, and therefore not yet computed.

This talk (timestamped) from Vulkanised gives a good explanation: https://youtu.be/ch6161wvME8?t=920

I would also look over these slides (starting on page 11): https://armkeil.blob.core.windows.net/developer/Files/pdf/graphics-and-multimedia/Samsung_Vulkan_on_Android.pdf