这个是be of + 抽象名词 ＝ be + 抽象名词对应的形容词
of是表示属性关系
比如：be of importance = be important
be of necessity = be necessary等等.
其他的be + 介词情况 都与此用法不一样.
比如：The book is on the table.
Your expection is over the ceiling.
Their unprecendented plan is beyond their ability.
这些介词都是表示范围的.