Starfleet has a wide range of vessel classes at their disposal. There are large, top-of-the-line ships like Galaxy-class and Sovereign-class vessels. The Intrepid-class is a lighter long-range explorer. When going into battle a Prometheus-class or Defiant-class ship would be a welcome addition to any fleet. If you are on a scientific mission to investigate some strange phenomena you’d want to bring a Horizon-class, and an Olympic-class flying hospital is the go-to ship for a medical emergency. The Starfleet ship class lineup can adapt to a wide range of situations, but it won’t stay the same forever.
Technology is always improving both for the Federation and other states. What was the newest technology available a decade before could already be surpassed by new technology. As the current ship classes go the way of the Miranda-class or Constitution-class new ship classes will need to take their place. If Starfleet came to you and put you in charge of designing the newest addition to Starfleet’s vessel classes what kind of ship would you bring back to them? Perhaps you think there’s a shortage of ships built to defend the Federation. Others could favor smaller ships as opposed to building larger and larger explorers that outclass the current flagships.