Why do spindles move chromosomes pairs to poles in anaphase I?

for your first question, WHY do spindles move chromosomes to the poles of the cell? well you need to separate the newly synthesized DNA (ie. separate the sister chromatids) to create two genetically identical daughter cells. HOW, its done is more involved. Tightly wound DNA fibers contain centromeric DNA, which a protein complex called the kinetochore binds and links this DNA complex to a long microtubule (the spindle!). Through the action of a motor protein (kinesin) which creates a "pulling force" at the kinetochore when ATP is hydrolyzed, the chromosomes "walk" towards the negative pole of the microtubules (the cell periphery). These steps are very tightly regulated, and any change immediately targets the cell for death via apoptosis.