One of the first things that is covered in a PDE class (and linear algebra, of course) is the concept of linearity and linear operators, i.e. an operator $L$ such that $L(c_1f_1+c_2f_2)=c_1L(f_1)+c_2L(f_2)$.
It is then said that $\frac{\partial}{\partial x}$,$\frac{\partial^2}{\partial x^2}$, etc are linear operators because they fit these definitions.
What is the importance in this? Does it have something to do with the superposition principle? I haven't solved an equation written as $Lu=0$, but rather we are given $\nabla^2u=0$, and although they are in the same form, I don't see the importance in bringing up the fact that those things are indeed linear operators.