Can reading a value of a variable kill the definition of the variable?

I was going through the concept of reaching definitions from the red dragon book.

The authors define reaching definitions as follows:

Definition: We say a definition $$d$$ reaches a point $$p$$ if there is a path from the point immediately following $$d$$ to $$p$$, such that $$d$$ is not "killed" along that path.

Then the authors consider a flow graph and explain the definition with an example as follows:

For example, both the definitions i:=m-1 and j:=n in block $$B_1$$ in Fig. above reach the beginning of block $$B_2$$, as does the definition j:=j-1, provided there are no assignments to or reads of j in $$B_4$$, $$B_5$$, or the portion of $$B_3$$ following that definition.

I can understand that in $$B_4$$, $$B_5$$, or the portion of $$B_3$$ following that definition j:=j-1 if there is an assignment to j then the definition $$d_5$$ is killed. But what problem shall be there if there is simply a read of the value j? How does it kill the definition $$d_5$$ and prevent it from reaching the top of $$B_2$$?