Question

why is absolute ruler important?

Answer 1

important benefit of absolute monarchy is that it can bring more stability to a country than a more feudal system often does. In a system where the monarch's power is less absolute, we often see terrible civil wars that arise as various of the more powerful lords jockey for power and try to position themselves, if possible, to take the throne. This is much less likely to occur in a country where the monarchy is more absolute and the power of the lords less relevant.
In addition, the absolute power of a monarch can be advantageous if the monarch is a good one. In such a case (rare though it may be) the monarch has a free hand to act as they wish without having to contend with obstacles set up by lords or a Parliament.

All this said, absolute monarchy does not seem on balance to be a very beneficial system.