Over time, dust and dirt can build up on the internal mechanisms that control the movement of a lock. You can try applying dry graphite to lubricate it or if you think it might be more than just dirt affecting it, a locksmith can assess the mechanism and tell you if you need a new lock.
Wooden doors can also expand and contract which stops the lock from fitting into the door frame. This can happen if the lock was installed during hot weather or in the middle of winter. Once the temperature changes, the door can shift slightly. In most cases, the lock will still work if you wriggle the key around and get it engaged, but you can ask a locksmith to adjust the lock for you to stop this from happening.
You won't always need to replace your set of locks if your keys are stolen. If you choose to replace your entire lock, the lock itself and the knob will be replaced but it can be cheaper to have your locks re-keyed. In a re-keying job, the tumbler's replaced so that you keep your old lock but only a new key would be able to open it. Your old keys would stop working in the lock.
There is no legal obligation for locksmiths to be insured but locksmiths may choose to purchase public liability insurance so they're covered in the case of injury or accidental damage to a property.