Yes and no. Saying what you want, whether in your head, or visually, is important part of spellcasting. Visualization is used to guide the energy to the desired purpose. If casting a love spell, you might want to visualize a pink-red energy enveloping your target. Without doing the visualization, the direction of the energy might be skewed.
Beyond the typical trio of "Will, Intent, and Action" there are another three pillars that are summed up into visualization, to complete your workings.
These are; Belief, Emotion, and Imagination. For one must believe in their power, know it. They must be emotionally invested in the outcome, and be able to internally envision and live through that outcome.
These together is what we call and why we use visualization.
Think of it like this.
I can decide I want to paint something specific, let's say, a portrait. As an artist, we must do the painting, but without being able to see what we're going for, both in person and in our minds eye, we may end up with very different results than we intended.