This is done to increase the conductivity of the solution (to practically eliminate the so-called IR drop, or ohmic potential drop from Ohm's law: V = IR), to eliminate the transport of electroactive species by ion migration in the electric field, to maintain constant ionic strength, to maintain constant pH, etc.
The reason is not to be searched in its thermodynamic stability because when in contact with a reducer at high temperature, it violently reacts to dissipate a large quantity of energy in a vigorous exothermic reaction.
The reason of its redox inertness when dissolved in water is due to severe kinetic limitations to abiotically accept electrons, even if the oxidation state of the central chlorine atom in this tetrahedral oxyanion is +7.
In term of chemical kinetics, perchlorate is a non-labile species because of a high activation energy hindering its redox reactivity.
Hypochorite (ClO−) and chlorate (ClO−3) anions although being able to accept less electrons than perchlorate (ClO−4) are much stronger oxidizers in aqueous solution because of less kinetic limitations.