Like a carpenter’s measuring tape, it’s right there on your tool belt to assist you when you need it.
We’re not measuring inches or feet, however, but on-screen pixels. Measure widths and heights, gutters, make sure something is perfectly center, you name it. Also helps out in graphic design projects. A good designer is pixel perfect.
For such a simple tool, Screen Calipers are extremely useful. They probably get used once a day at least, similar to other tools that have become daily staples: Notepad, Photoshop, and Firebug. It’s an important tool to check your work as well on the front-end, just like you check the back-end with the W3C Validator. They are essential.
If you’re a new designer, I highly recommend Screen Calipers: