How are useless states created NFA to DFA
Below a nondeterministic automaton for the language of all strings where the one-but-last letter is an $a$. If we make it determinsitic using the standard construction half of the states will be unreachable.
You ask, why does this happen? Look at the initial state $0$ of the original automaton. It has a loop for both $a$ and $b$. That means that in the powerset construction all next states will always contain state $0$ because the deterministic automaton collects the states of all paths from the original automaton. As a consequence all states without $0$ are uneachable.

It is possible &quot;by hand&quot; to avoid those unreachable states, by a step by step construction. Only consider staes that are reached and continue form those to the next possible states. A kind of depth-first construction will work.