Structured content is organized in a specific way that caters being used
repeatedly. It helps structure data in a way that computers and devices can
recognize easily.
Semantic HTML is associated with HTML, it uses specific elements
such as tags to help describe and make it look readable. This way, humans can understand
are able to interact with machines as well.
Structured content, like the name suggests, organizes the content. Semantic HTML then uses that structure and presents it a way that allows humans and machines to easily identify.
SEO can heavily be influenced by semantic HTML since it makes it easier for search engines to find and rank the content. Another reason can be just overal usability across devices, since you using the same structure and adding similar tags across devices. This practice can also argue for future proofing since our devices will evolve with emerging technologies.