Garden drainage

Hi all. New member here. Not sure if I'm in the correct place for this. I have attached a picture of an area in my back garden just below the kitchen window. My garden has always had bad drainage and the water has often pooled here for a while. Recently I have noticed that the water stands here for a lot longer than in the past. I've never really thought about this area in the past. Does anyone recognise what this may be? I assume there must be a reason that it wasn't slabbed like the adjacent area. On the left there is a hole in the ground that looks like a drain. It was full of goopy mud and rocks. I cleared some rocks out and the water in the hole eventually drained but new water still stands for a long time. The water is muddy at the moment as I am laying some new chips down in the garden so there is a bit of mud kicking around. Usually it's clear rainwater. Anyway if no one can help then fair enough. Not really sure what to search for. It must serve some sort of purpose. Thanks in advance for any help offered.