Graphs aren't mandatory, but they may help visual learners see why an equation works a certain way. Also, they are a visual tool to quickly find a solution to a problem. Furthermore, they help with presentations to quickly convey an idea to someone (who may not be well versed in mathematics).
For example, if you save $10 a week and already have $50 in a savings account, then the equation you graph is y = 10x+50. Here x is the number of weeks and y is the amount saved.
Now let's say you want to find out how many weeks it would take to have $90 total. You could use guess and check to get the answer. Or you could use algebra and the substitution property. The visual way would be to graph y = 10x+50 and y = 90 together to note how the two lines intersect at (4, 90) to indicate it will take x = 4 weeks to have y = 90 total.