Hydraulic Top Links are simply used for tilting implements back to forward.
One of the more common reasons for this would be a box blade. With a box blade, when you extend the top link, the rear blade is the first to contact (dragging) and when you retract the top link, the shanks are the first to contact (ripping). Somewhere in between these two extremes, you get a combination of the shanks and the box blade. This is a very basic explanation. I can tell you that after using my box blade for several hours with a mechanical top link, I have decided that I need (want) a hydraulic top link.
It is also useful for disc harrows to change the way the discs engage with the ground.
It is VERY useful if you have an iMatch or other type of 3PH quick hitch. You usually need to extend or retract the top link slightly to change the iMatch angle, depending upon the implement.
It is useful for mowing up bankings with a rotary cutter or finish mower. Simply retract the top link and then back up to the incline. You could also extend the top link and back up to the decline.
I'm sure there are several other reasons why you might want one. I will let some of the other folks chime in.
Assuming you have a free SCV (usually a 3rd SCV), they are very easy to install. You simply replace your mechanical top link with the hydraulic top link. You then connect the hydraulic hoses to your 3rd SCV ports and you are good to go.