why do some people use screws when building things while others use nails? any difference with structural integrity?

If you are talking about woodworking, screws have a tremendous advantage. They have greater holding power. They are less likely to work themselves loose. Almost every place that I used to use nails, I now use screws.(I don't have a power nailer. I don't do framing or I would use a nail gun.) The biggest reason for using a screw is a power screwdriver. This has also changed the screw heads that I use. A Philips head screw while fine with a screwdriver will cam out with a screw gun. I usually use a Robertson square drive or Torx screw to avoid this.

Screws hold tighter and in many cases are easier to use for assembling furniture, shelving, etc. They can also be easily removed. Nails are more frequently used for large assembly and finishing that does not require extra precision, like roofing shingles, wall framing, wood panelling.

screws will hold tighter and they do not pull out === nails will just pull out if there is wind force or very dry conditions and nails will rust ......... Screws will always be the best to hold things together and the use of dry wall screws is good because they do not rust .................. the above parameters describe the differentiates of the materials as they bear on structural integrity ...........

Well, sometimes nails are better because they disturb the material less, sometimes screws are better because they have more contact with the materials.

It depends on the kind of stress the fastener will be withstanding