Because when a function is differentiable we can use all the power of calculus when working with it. Continuous. When a function is differentiable it is also continuous. Differentiable ⇒ Continuous. But a function can be continuous but not differentiable. For example the absolute value function is actually continuous (though not ...

Differentiable. A differentiable function is a function in one variable in calculus such that its derivative exists at each point in its entire domain. The tangent line to the graph of a differentiable function is always non-vertical at each interior point in its domain. A differentiable function does not have any break, cusp, or angle.