There is no purpose to "sag" It is merely how much the springs compress under the static weight of rider and bike. It has become a somewhat misunderstood measure of preload on the suspension springs. A general rule of thumb that becomes less useful with riders of different weight or springs that have sacked over time or been changed for accessory springs of a different rate.
I was planning on going into this in depth at some point with pictures and diagrams but I'll try and cover a pocket version here.
On a typical suspension application the springs and the weight of the bike/rider form a low frequency filter. As the wheels move rapidly to follow the terrain the frame reacts slower. The idea being that the quicker wheel movements are averaged out providing a smoother ride. At the same time the wheel weight and springs form a second resonant filter based on wheel weight and spring weight. This determines how fast the wheels react to changes in the terrain to stay in contact with the ground. The two systems are somewhat counter to each other as a lower spring rate/heavier chassis gives a smoother ride but a higher spring rate/lighter wheel maintains better contact with the ground. This is where "unsprung weight" becomes critical. Unsprung weight is essentially the wheel and any suspension components that are directly connected to the wheel on the bottom side of the shock. The lighter the wheel the lower the necessary spring rate to keep the inertia of the wheel from making it lose contact with the ground.
Trials bikes are usually set up with very soft suspension so the first thing Clydesdales like myself notice is how easy it is to bottom out the suspension. The cheap answer is to crank up the preload. It's not the best solution.
Preload is not spring weight. If I have an unloaded spring that takes 50kg to compress a centimeter it takes 100kg to compress it 2 centimeters, 150kg 3 centimeters. If I have the same spring preloaded 1 centimeter and I put 50kg on it won't compress. If I put 100kg on it will compress 1 centimeter, 150kg 2 centimeters. I've effectively lost the first centimeter of travel. What that means in practical terms is if I ride over a series of little stutter bumps that never put more than 50kg of load on the spring it might as well be a solid metal spacer. If you've ever driven a 1 ton truck with nothing in it around a bumpy corner you know the feeling well as all it does is skip across the road.
Assuming you are not built like Fujigas a heavier spring with less preload is better for two reasons. The first is less preload means the spring will actually react over smaller bumps. The second is peculiar to trials and specifically any maneuver that stores energy in the suspension to be released with the wheel leaving the ground, splatters, zaps, hops... With the spring preloaded you have to put in much more energy to get the suspension to move but you don't get it all back. If I jump on the above mentioned preloaded spring with 150kg of force that first 50kg gets transferred directly to the ground and only 100kg of energy is available to be recovered from the spring. OK it's more complicated than that as energy is stored in the tire but that's the general concept.
So sag is not an absolute measurement. More critical is balance between the front and rear suspension, how well your wheels are tracking on bumpy terrain, your personal riding style. Big hits and trick riding vs. slow turns and bumpy sections. If you can find any video of Jordi Tarres and Eddy Lejuene you will get an example of how different two world champions suspensions can be. Jordi's bikes were always sprung very high with minimal damping where Eddy's bikes were soft as a sneaker full of oatmeal.
If you're bottoming out a lot a little preload goes a long way with a rising rate suspension but be careful as it's very easy to dial in too much. Best approach is small adjustment, ride, small adjustment, ride. You will eventually find a setting that works for you.
