Why do plants in water-logged soil have to use anaerobic respiration?

Aerobic respiration is when a cell uses oxygen molecules to generate usable energy for itself. Anaerobic respiration is when a plant is using something besides oxygen molecules to generate usable energy. The reason a plant submerged in water needs anaerobic respiration is because the plant has no oxygen molecules available. 
Plants prefer aerobic respiration, as anaerobic respiration provides much less usable energy than its oxygen powered counterpart, but in a pinch can resort to anaerobic respiration. 
Other creatures can use anaerobic respiration as well. Yeasts, for example, produce alcohol when engaging anaerobic respiration. This is one method used to make alcoholic drinks.
Some other organisms use anaerobic respiration as their primary method of generating usable energy, like archeabacteria. Rather than oxygen, these processes use chemicals like sulfates and nitrates.