A lower hourly rate does not automatically mean a lower project cost. Engineering experience affects how quickly problems are identified, solved, and prevented.
Senior engineers often spend more time clarifying requirements and evaluating simpler solutions before implementation. Those decisions can prevent significant rework later.
Experience also matters when products move beyond prototypes. Architecture, performance, security, testing, and maintainability become increasingly important as real users arrive.
This does not mean every project needs only senior engineers. Strong teams combine experience levels with appropriate technical leadership and clear ownership.