The original answer I'm improving on is shown below and is somewhat simplistic.
A locksmith is a security professional who does a great deal more than "Pick locks on doors". In fact, except for those locksmiths who specialize in emergency work only, picking locks is NOT a major part of a locksmiths business.
A locksmith:
Knows how to re-key locks and when it is appropriate to do so, repair or replace broken locks, install new locks, formulate master key systems, install and repair door closers, cut multiple types of keys.
Knows life safety and fire regulations as pertains to doors and locking systems. Usually can repair doors which are not opening and closing properly. Usually can change safe combinations.
Knows how various locking mechanisms operate and how to defeat them if necessary.
Knows what hardware is available, and what hardware is compatible with other hardware.
In 25 years of doing this job, i am always amazed at how people will got to a home center and buy a $14.00 knob and deadbolt set and call and ask me to remove their heavy duty knob and deadbolt set that costs $400.00 to put it on. They can't tell the difference between a $10.00 cheapo lock and a heavy duty lock that will actually protect them. There's a reason why the cheap locks are so cheap! Is that what you want protecting your jewelry?
People also tell others to buy new locks at a home center and save the cost of a service call. What they don't know is that the locksmith will know if installing certain locks will cause a fire escape hazard.
they pick locks on doors to help people who are locked out of somewhere