Loop of HenleWatch
basically removal of majority of water occurs in proximal convulated tubule however, loop of henle has the main role to reabsorb some water by pumping salts against concentration gradiant into tissue fluid. if tissue fluid has low water potential before pumping occurs, a lot of water will be reabsorbed. however, if there is a high water potential in tissue fluid even when ions are pumped, only a small quantity of water will be reabsorbed.
That's its main role, which involves reabsorbing water in the descending limb and actively pumping out NaCl in the ascending limb. This then makes the tissues surrounding the collecting duct more negative than the inside so water moves out by osmosis. That wouldn't be possible if the kidney had a constant water potential.
I can see you're still a bit confused so let me try to lay it out simply for you :-)
The role of the entire kidney is to filter the blood and remove waste of metabolic processes through excretion.
The role of the Bowman's capsule and glomerulus is ultrafiltration.
The role of the PCT is to selectively reabsorb glucose, water and amino acids.
The role of the LOH I described above.
The role of the DCT is not covered in my OCR spec, but it basically regulates the conc. of ions.
The role of the collecting duct is to transport the urine to the bladder.
Bear in mind, these are MAIN roles, and many other specific processes occur that you'll need to explain in an exam. Hope this helps!!