Forms of life cannot survive if they do not reproduce. Different species have different means of reproducing, and different means of reproduction have different advantages and disadvantages. Asexual reproduction is the most common type.
All forms of life eventually die, and species depend on reproduction to survive. Asexual reproduction, where an individual reproduces by itself, is the most common type. Bacteria and other single-cell life forms simply split into two. The rate at which bacteria can reproduce is high, and they are only limited by available sources of energy and available space.
Viruses cannot reproduce on their own, which is part of why many scientists don't consider them to be alive. Viruses infect other cells, and they hijack their internal systems to force them to create more viruses. Eventually, the cell dies and releases the viral bodies it created. This form of reproduction has proven to be tremendously effective.
Sexual reproduction, which requires two members of a species, is common in the animal world, but it is also used by plants. Bees and other creatures carry pollen from one flower to another. Sexual reproduction allows two individuals to combine their genes, which helps pass on beneficial traits and aids in evolution.