Texture designs are an important part of any realistic drawing. Unlike shadows who are giving a sense of depth, textures will give to an image a feeling of roughness (or smoothness... it depends!).
And you don't need to take a picture of a piece of metal and add it to your image to make the whole thing interesting.
Like you will see in the image below, a nice and well-done texture can be made using a simple pencil and lots of imagination and patience.
I have created this comic strip a couple of years ago, when drawing images on a computer was not possible! All I had back then was a pencil, a marker and a piece of paper.
Drawing a comic strip with this level of details was a long and difficult task but at the same time, it was really rewarding.
Now, using a simple drawing software, it is possible to create more complex images in less time and effort. Textures are not a must when creating a comic strip.
You can exclusively use shadows and still end up with a nice results. But adding textures will, without a doubt, add more realism and professionalism to your work.
Take a minute to analyze the picture below to see how you could use simple patterns to create nice and effective textures!