Concept:
Reserved antibiotics are antibiotics kept for special situations and are not used routinely.
Step 1: Why antibiotics are reserved.
If powerful antibiotics are used unnecessarily, microorganisms may develop resistance.
Step 2: Purpose of reserved antibiotics.
Their restricted use helps preserve their effectiveness and prevents the emergence and spread of antimicrobial resistance.
Step 3: Why other options are not correct.
They are not meant for minor infections.
They are not used to increase hospital revenue.
They may be used in severe infections, but the main purpose of reserving them is to prevent resistance.
Step 4: Final answer.
Thus, the main use of reserved antibiotics is to prevent emergence of resistance.
\[
\boxed{\text{Prevent emergence of resistance}}
\]