Managing without the command line.
WHAT A CONTROL PANEL GIVES YOU
A web interface for sites, email, databases, DNS and files.
Broadly what you had on shared hosting.
WHY PEOPLE USE ONE
Most of the convenience of shared hosting, with the control of a server.
WHAT IT COSTS
A licence fee, for most commercial panels.
Some free options exist with fewer features.
WHAT IT DOES NOT REMOVE
Responsibility for updates, security and backups at the system level.
A control panel manages your sites. It does not manage your server.
WHEN TO INSTALL IT
Early, on a clean system, before other software.
Panels expect to configure services themselves and conflict with existing configuration.
WHAT TO CONSIDER
Whether you need it. If you run one application and are comfortable with the command line, you may not.
FOR RESELLERS
Almost certainly yes. Managing client accounts without one is impractical.