Most practical reasoning is hypothetical in form: if you want health, exercise; if you want wealth, work diligently; if you want happiness, cultivate friendship. These commands bind only someone who has the relevant goal. Change the goal and the imperative lapses. They carry no unconditional authority — their force is entirely borrowed from the ends they serve.
A categorical imperative commands unconditionally. It does not say "do this if you want some result" — it simply says "do this." Its binding force does not depend on your inclinations, your ends, or the consequences you anticipate. It concerns only the form and principle of the action itself, and this is what makes it capable of grounding morality in something genuinely necessary and universal.
If moral commands were hypothetical, they would bind only those who happened to share certain goals. Morality would dissolve into a set of prudential recommendations — useful advice for those who care about certain outcomes, but with no authority over those who do not. Kant insists that genuine moral obligation must command all rational beings simply as rational beings, not conditionally on their particular desires.
Kant introduces this distinction in the Second Section of the Groundwork as part of his analysis of what kind of imperative morality requires.
