The information architect is very much like an architect in home or commercial real estate building. They have experience in various aspects of the build of the structure. From design & UX to development and project management, the architect will be able to take your ideas and envision what should be built to accomplish your goals.
After they have an understanding what you're building, the architect, very often along with the assistance of a team of developers and designers can help you plan an information architecture. Keeping with our analogy to a physical structure, the information architecture is the blueprint for your building. It outlines every section, subsection, feature, functionality, and anticipates issues in growth or change to the systems.
Without this document, your build team would try to build a house without a blueprint, and that is a terrible idea.
Just like in life, there is no substitute for experience. The best Information Architects have been doing this job for a long time. When hiring an information architect, be sure that your architect has worked on similar systems, is familiar with the technologies you need to use or can consult you on those technologies.
The best architects I have worked with have experience as a designer, developer, and project manager. They may not have been great at all of those roles, but they certainly worked long enough in the industry to have the experience.
If you choose a less experience architect, remember that you learn by making mistakes. Don't be someone's mistake.