Let's consider a death mage as an example of using the calculator. We will set our target attunement to 100. In the first screenshot we achieve this attunement by manually locking the ten lowest cost death skills to 100. This means that no optimization is going to occur because all the skills are locked into place. We see that achieving 100 death attunement in this way costs 34.7 million exp.
Now let's use the calculator to show us how to achieve 100 death attunement with the minimum possible exp cost. Here we'll let the calculator choose the skills for us, and notice how it still chooses the same ten skills from the last example. Now the skills are all free to be optimized, and it tells us what level we should make those ten skills. Notice that with this configuration we've achieved 100 death attunement for only 30.7 million! That may not be a huge percent difference, but I'll take four hours of my life back from grinding exp any day.
Finally, your build may not want to focus only on the ten least expensive skills in the magic school of your choice. Notice that Summon Lich was not amoung the skills used in the previous two examples. Also, you may want more expensive skills to be higher than less expensive ones. For such an example let's consider a death mage who wants to focus on summoning lichs. We will drop Summon Skeleton from our skills to consider, and add Summon Lich. We then lock Summon Lich, Undead Mastery, and Mummified Undead to 100 each, because we don't want these skills to change in the optimization. The calculator then tells you what your other seven skills should be set to in order to achieve 100 death attunement with the minimum amount of exp under these constraints.