Coherence

I want to give an intuitive idea of coherence. One must be having an idea of coherence meaning a constant phase difference, but what is a phase? There is, of course, a dictionary meaning of the word 'coherence'. However, in physics frame, we want something more sciential.
From our understanding of wave optics, we know that two coherent waves interfere to produce fringe patterns on the screen. But why is it important for the waves to be in phase. The idea lies in the fact that, the interference pattern is a result of the vector addition of the amplitude of the two waves. With algebraic addition, we return to the classical picture to get uniform intensity. With reference to electromagnetic waves, we know that EM waves comprise of electric and magnetic vectors. Now, if two vectors do not maintain a constant phase, that is, relative orientation in space with respect to each other, then they are, in fact, not to be designated as vectors at all! Consider this, if two vectors continuously change orientation with respect to each other, then it simply implies that they don't have a definite direction at all. What good a vector is if it doesn't have a definite direction!
Thus, it gives us an idea why for interference we need coherent waves, that is to say, a constant phase difference.