Using a large needle on delicate fabric, for example, will cause visible holes - not a good look - and may cause problems such as the needle shoving the fabric down into the bobbin casing - arrghh . You might also find that the stitches slip - and your project.. well.. you can start again!
On the other hand, using a small needle on heavy fabrics can cause problems such as the needle breaking under the pressure - how many times did that happen to you?
BEHIND THE NUMBERS
Sewing machine needles come in different sizes. Confusingly, two systems of naming the sizes run side by side: imperial sizes go from 8 to 19 - 8 being a fine needle and 19 being a thick heavy needle, metric sizes from 60 to 120 - 60 being a fine needle and 120 being a thick heavy needle.
All my packs - except the French one - are marked with both the imperial and metric sizes.
I was so happy to find that table!! Source
There's a wide range of needle types to choose from, depending on what you're sewing:
Universal - The needle's point is slightly rounded for use with knit fabrics, but sharp enough to pierce woven fabrics
Denim/jeans - This type of needle has an extra-sharp point and stiff shank, which makes it suitable for stitching denim, heavy imitation leather or other densely woven fabrics
Ballpoint - This needle has a blunt, rounded tip that slips between fibres rather than piercing them. Use this needle for sewing coarse knits, lycra and other fabrics that tend to run if you snag them
Twin needle - Twin needles are constructed with two shafts on a crossbar which extends from a single shank. They're ideal to use for decorative stitching and creating multiple, uniform stitching rows. Two numbers are listed on the packaging; the first number represents the distance between the needles and the second number is the metric needle size. (More on threading a twin needle here)
MORE THAN MEET THE EYE
In addition yo the difference in the shapes of the point the needle eye changes as well, and choosing the correct one for your projects depends on the type of fabric or the thread being used.
Changing the needle on your sewing machine (like now!). Needles can blunt quite quickly, especially if you're sewing synthetic fabrics. Blunt needles will not only damage the fibres in your fabric, possibly snagging it, but will also cause skipped stitches and occasionally puckering.
Always test your thread and fabric combination on scraps of fabric rather than the item you are working on. In my case it is as much to see if the needle is appropriate as to see if my machine is well threaded.