If you write and distribute your own custom handler or buy a third-party handler for your own system, it's likely that a setup program will be distributed. If not, the agent must be registered manually, and its files copied. In either case, a server administrator is the person to set up the agent.
Similarly, if you're a folder author and you want to install a new agent that uses the handler you've written, you need to request that your server administrator install your handler.
For more information about administrators' roles, see Microsoft Exchange Server Administration and the Microsoft Exchange Server Getting Started Guide.