Why navies repeatedly develop light, medium, and heavy ships

Does a navy need a cheap security tier, a moderate mass tier, and a premium command or specialist tier?

Broadly yes. Fleets repeatedly develop different sizes for different tasks, though the exact categories vary by period and the lightest vessels often serve presence missions.

Different sizes and costs of warship recur across naval history, although the boundaries do not fit every period neatly. Ancient fleets included small, fast galleys and exceptionally large vessels whose fighting value could be questionable but which offered room for command. The 74-gun ship provides a later example of a useful middle size. An equivalent middle category for World War II is left uncertain here, with cruisers suggested tentatively. The lightest tier covers tasks where a navy needs a ship present even when substantial fighting power is unnecessary. When those vessels operate with a fleet, they still need enough speed to avoid being left behind.