The main reasons why cell division is important in organisms are reproduction, repair and growth. Cell division is necessary in order for life to continue.
The length of the cell cycle is important because it determines how quickly an organism can multiply. For single-celled organisms, this rate determines how quickly the organism can reproduce new, independent organisms. Most of the differences in cell cycle duration between species and cells are found in the duration of specific cell cycle phases. DNA replication, for example, generally proceeds faster the simpler the organisms. One reason for this trend is simply that prokaryotic have smaller genomes and not as much DNA to be replicated.