Why is it useful?

It may be more appropriate to replace the word useful with necessary. Complex requirements, large numbers of components, legacy systems, cross dependencies etc. In the same way that city planning or building design necessitates an architectural type role so too does providing modern IT solutions.

The impact of altering design s made in the early stages of a project can have a disproportionately negative impact on a project delivery. For example, selecting the wrong tool for a case management solution. Integration work, user interface design, workflow, database setup, reporting. The list goes on and on.

Call it Solution Architecture or something else. The fact remains that managing these levels of complexity requires a dedicated role. A cross section of skills including developer, analyst and project manager. Today we call that discipline Solution Architecture.