Some libraries have a dynamic and static part. An ld script loads both libraries in the correct order with the INPUT command, for example:

INPUT(libstatic.a)
INPUT(libdynamic.so)

This script instructs the linker to load libstatic.a then libdynamic.so

An ld script can be given to control how the linker combines sections, for example:

SECTIONS
{
    .text :0
    {
        *(.text)
	*(mysection)
    }
}

This script instructs the linker to combine mysection and all the .text sections into a single .text output section.

You might want to combine sections together in a different order to that given by the default linker script. Also, you might want the linker to define symbols at specific addresses. This information can be given by a custom linker script.