Why do some organisms use both types of reproduction

You're talking about sexual reproduction and asexual reproduction, right?
Well, when there are favourable conditions they use the asexual reproduction because they want to produce as may individuals as possible with the lowest energy use possible. 
When the conditions are no longer favourable, they use sexual reproductions, because the egg is more resistant to the unfavourable conditions and is more likely to thrive.

Wetlands form where water and land meet. a lake with a wetland fringe can fill in with decayed plant and animal material to become a wet meadow. a pond can fill in to become a bog or fen. sediment can fill in a river marsh to become a swamp.flooding of coastal lowlands from rising sea levels has created broad coastal marshes that are protected from wave action by barrier islands or reefs. coastal wetlands also form when rivers deposit sediment as they reach the ocean. plants then take root and hold the soil firm against the forces of tides and waves.