According to Kirchhoff, I wonder about heat storage in general. When everything that absorbs radiation well, emits it also well, so what is the point of using certain materials for specific purposes then? I know this is quite vague but I lack applied examples at the moment. What I'm trying to express is: For example a lake. The sun is heating it up and it is definitely great at storing this heat. But does Kirchhoff mean that a single atom(?) would emit the received energy immediately/soon and in a lake we don't experience this because there are so many atoms picking up this emitted energy from another atom?
Firstly, Kirchhoff's law applies to bodies that are in thermodynamic equilibrium with their surroundings. Heat storage devices are, almost by definition, not in equilibrium with their surroundings - either they are accumulating heat or they are emitting heat.
Secondly, Kirchhoff's law is a macroscopic law that describes the bulk properties of bodies that are collections of (at least) millions of atoms. You cannot apply it to the behaviour of a single atom.
Kirchoff's law is about rate of radiation transfer at thermal equilibirium. So one can make big bodies to store more energy or lose less energy, because large body has small surface area. This is applied in where there is passive or active cooling is needed. The shape and design of air cooled engine's casing has thin metallic strips like structure to increase surface area, thus increasing rate of transfer of heat from engine. This same design used in motherboards where fan is not available.
In animals, an elephant require more cooling because its volume is much large. Stefan-boltzmann radiation law is based on this and Planck's law got inspiration from it. The definition of poverty line once deduced from Stefan-Boltzmann law by monetary institutes.