[Bloc] Why space is weakly referenced?

Just to understand

space
     "Return space to which I belong. Direct reference boosts performance.
     Each element has weak reference to the current space which is set
at creation moment.
     @see BlElement>>#basicSpace:"

     <return: #BlSpace>
     self assert: spaceReference isNotNil.
     ^ spaceReference ifNotNil: [ :sr | sr at: 1 ]