TITLE: Why a quadratic equations always equals zero?
QUESTION [10 upvotes]: On evaluating quadratic equations, It always equals zero:
$$ax^2+bx+c=0$$
Why zero? Is it possible to use other number for another purpose?

REPLY [2 votes]: I FINALLY UNDERSTAND WHY THE QUADRATIC EQUATION IS SET TO ZERO!!!!
You "solve" a quadratic equation by figuring out "WHEN Y=0 what does X equal?"(x,0). That's why you set the equation to 0 and not any other number; to find the X-intercept(s) aka (x, 0) point(s).  
HERE'S WHY:
A quadratic equation can be thought of as a function ("f(x)=" is a fancy way of saying "Y=") ; the point being you have an equation where Y = ax^2 + bx + c
***THE KEY IS THE "Y =" PART!!!
When you're asked how many "solutions"  this type of equation has, it's really asking: how many times does the graph (the Parabola in this case) intersect the X-axis? 
You "solve" a quadratic equation by figuring out "WHEN Y=0 what does X equal?"
That's why you set the equation to 0 and not any other number; to find the X-intercept(s) aka (x, 0) point(s).  
The ROOTS of a quadratic equation exactly means the x-intercepts ((x,0) values)
*NOTE:
No solutions means the parabola does not intercept the x-axis (maybe the parabola is above the x-axis or completely below it)
2 solutions (which is the most common for parabolas) means there are TWO places the parabola crosses the x-axis. 
Finding the solutions means finding the (x,0)point  value(s)
Hope this conceptual understanding helps you as much as it did me! :)