In the definition of an $n$-dimensional stratified pseudomanifold one demands the following filtration $X=X_n \supset X_{n-1}=X_{n-2} \supset X_{n-3}\supset ... \supset X_0 \supset X_{-1}=\emptyset$. where all $X_i$'s are closed subspaces of X. Why is it required that $X_{n-1}=X_{n-2}$?