Hard inquiries are those that are initiated by a merchant when you request credit and yes they do ping your credit score slightly. Soft inquiries are those that are initiated by you, ie your free annual credit report from one of the 3 bureaus, and do not affect your credit score.
But to answer your question, I guess they have to enforce a slight penalty for hard inquiries so people with horrible credit can't simply play a numbers game and apply for an infinite number of lines of credit. Even a person with horrible credit will eventually be approved. I guess they want to avoid this horrible credit score from being approved.
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