Those of us guitarists who play every day, no matter how good we are, notice one consistent thing: Inconsistency! No matter how burning my playing was yesterday, there are times when I pick up the guitar the next morning and I'm struggling to make it work for me. That's when having multiple instruments to chose from really helps. When my Strat doesn't work, I can pull out a different one, or maybe a Les Paul or a Tele for a while. Playing my SG or ES-335 brings out an entirely different side of my playing, and it might be just the break I need from my usual tones to inspire me that day.
This holds true whether I'm working or just practicing. Scale lengths vary, sizes differ, and neck width and even string-spacing changes from guitar to guitar. When you're young and learning, the tendency is to sell your one and only guitar to obtain the money for another, newer one. But as soon as you can afford to break that chain and keep the one you have while also acquiring the new one, do it!
Besides building a wide palette of tones you can choose from in your own personal musical direction, you'll have options for more working situations and your practicing time can be more rewarding. —Carl Verheyen