Sexual reproduction produces variation in the offspring. If the environment changes, for example because of a new disease or a change in climate, this variation can give some individuals a survival advantage by natural selection. Humans can speed up natural selection by selective breeding to increase food production. Sexual reproduction needs two parents, so the organism must find a mate, which takes time and energy.
Asexual reproduction needs only one parent. It is more time and energy efficient because the organism does not need to find a mate. It is also faster than sexual reproduction, and many identical offspring can be produced when conditions are favourable. The disadvantage is that there is no variation, so a change in the environment may harm all the offspring.
Some organisms use both methods, depending on the circumstances. Malarial parasites reproduce asexually in the human host but sexually in the mosquito. Many fungi reproduce asexually by spores but also sexually to give variation. Many plants make seeds sexually but also reproduce asexually, for example strawberry plants by runners and daffodils by bulb division.