Answer
The sister chromatids are kept together so that after the first and second cell divisions, every daughter cell has an equal amount of the respective parental homologs. Imagine for example, how much easier it would be to find your socks if you bound them at the ankles before you threw them in the wash. In doing so, it would make it easier to sort the laundry as the socks would already be together. This same principle can be applied to the chromatids, with the sister chromatids as the socks and the bivalent as the knot that binds them together, making them easier to sort into the daughter cells, or folding pile.
Work Step by Step
If the parental homologs were not paired with their sister chromatids then there would be no way for the meiotic spindle to discern which of the chromatids belong together. If this was the case, the daughter cells would have an uneven amount of chromosomes and would result in birth defects or mental deficiencies.
