They are used to fasten two or more parts together and allow for maintenance and repair disassembly. The longer screws are fully threaded and are often called tap bolts. The point of hex cap screws is chamfered (beveled) to protect the first thread from damage and to facilitate entry into an internally threaded part such as a tapped hole or nut.
Often, Larger sizes are not manufactured in short lengths, just as smaller sizes are not available in long lengths. Size and length are specified in fractional inches; length is measured from under the head to the extreme point (threaded end) of the screw.