We have the reaction

[tex]\ce{A} \rightarrow \ce{B}[/tex]

in a discontinuous system with constant volume, the speed [itex]r[/itex] with this reaction reaches equilibrium is given by the formula

[tex] r = \dfrac{-\text{d[A]}}{\mathrm{d}t} [/tex]

Experimentally it has been observed that the speed of a reaction is a function of the temperature, through the kinetic constant [itex]k_r[/itex], and of the concentration of the reactants or of some of them, each high for a certain number obtained experimentally

[tex] r = k_r[\mathrm{A}]^x [/tex]

where [itex]x[/itex] is called the reaction order.

My question is this: since the reaction rate is defined by -d [A]/dt, where does the need to express it through the kinetic law?