What are the practical reason for states - even the most powerful ones - to allow limits to their power?
This is a great question. Here are few points that you may want to consider.
First, not all states limit their power. For example, think of North Korea. The government is controlled by a dictator and he has complete control.
Second, when states limit their power, they do so, because they realize the potential for corruption. As the old adage goes - "absolute power corrupts absolutely." Hence, if there are limitations, there is less likely chance of tyranny.
For example, in ancient Rome, there were two consul who ruled together. This was, in part, to limit power. In today's American constitution, there are several branches as well and for this reason there is a limitation to power.
In short, wise states limit power, because the alternative can be disastrous.