Final answer:
Multiple initiation sites for eukaryotic replication ensure quick and reliable genome duplication over complex and large eukaryotic chromosomes during the S phase of the cell cycle by facilitating the formation of multiple replication forks.
Step-by-step explanation:
The major reason for multiple initiation sites for eukaryotic replication is to ensure the entire genome is replicated quickly and reliably during the S phase of the cell cycle. Eukaryotic DNA, being larger and more complex than its prokaryotic counterparts, is packaged into chromatin and requires the replication to start at multiple origins to facilitate the process efficiently over linear chromosomes. This occurs via the formation of multiple replication forks at these origin sites, which then proceed bi-directionally until the entire chromosome has been replicated.