The element Nitrogen is essential to life and is the building block of proteins. Photosynthesis and plant growth is impossible without it. Although the Earth's atmosphere is an abundant source, it cannot be use by plants until it is 'fixed'.
This means converting gaseous nitrogen into forms usable by living organism. Usally cooperative bacteria that live in soil or in harmony with plants do the fixing. Life would not be possible if the nitrogen cycle was disrupted.
Azolla is a nitrogen fixing plant by symbiotic association with cynobacterium which fixes atmospheric nitrogen.