Clarifying Practicality and Usage of a DFA's Accepted Language I was reading up on DFA's and their accepted languages when I stumbled across this:

Let a DFA $M$ accept the language $L⊆Σ^∗$, DFA $M'$ accepts $P(L)=$ {$w∈Σ^∗|wy∈L$ for some $y∈Σ^∗$}.

Since any $wy$ has to be an element of $L$ in order for $w$ to be an accepted string, and $L⊆Σ^∗$, then wouldn't all states be accepted by $M'$?

For example, for machine $M'$ with the alphabet $Σ=$ {$0,1$}, let $w=01$. Since all values of $y$ also have to be in the alphabet $Σ^∗$, $w$ would have to be accepted. Let $y=10$, $wy=0110$, which is in the alphabet $Σ^∗$. Similarly, let $y=110$, $wy=01110$, which is again in the alphabet $Σ^∗$. 

The text mentioned some transition from $M$ to $M'$, but what would the use of $M'$ actually be? How is it any different than, say, $L=$ {$w∈Σ^∗$}? Would they share the same finite-state diagram?
Picture the DFA $M$ as a directed graph, as the usual graphical depiction. Then the final states of your $M'$ that accepts prefixes of the language accepted by $M$ will have all states from which a final state of $M$ can be reached as final states. It should be clear that not all states of $M$ qualify. In particular, any dead states of $M$ (non-final states that loop back on all symbols) won't be final in $M'$.