Rhizobium converts atmospheric nitrogen into the soluble form , which can be then taken in by the leguminous plants.However since Rhizobium cannot prepare their own food , it  establishes in the roots of the plant and provide them with nutrients. Plants in turn provide it with food and shelter. Thus there exists a symbiotic relationship between the Rhizobium and leguminous plants.
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Peas, beans etc. are leguminous plants or legumes which are plants which have root nodules. they help rhizobium bacteria as they provide them with shelter in their root nodules and food. thus legumes help rhizobium bacteria.
rhizobium in return provide help to legumes. they are nitrogen fixing bacteria and they change atmospheric nitrogen in soluble form(nitrate and nitrite) which the plants can easily take from soil to make protein.
thus they help each other as there is symbiotic relationship between them.
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