The spindle serves two roles in mitotic cell division. these are to separate the

Spindle fiber is made up of microtubules that originates from the centriole present at both the poles of the animal cell. Plant cells do not contain centriole but can generate spindles from centrosomes.

During the mitotic cell division, the spindle fiber attach to a specialized structure made up of a protein complex called kinetochore which is present on the centromere of mature chromosome. Then these spindle pull the chromosome toward the pole through their centromere.

These spindle fibers help to separate the chromosomes during nuclear division in mitosis but it also helps in chromosome editing by holding two sister chromatids at their position together.  

The separation of chromosome is necessary so that after cell division both the daughter cell contains genetic material.