According to the description, Siena's main target audience is "business experts, business analysts, consultants and other app imagineers."
Examples of the kinds of Metro-Style/Windows Store/modern apps that can be built using Siena include apps for navigating "media-rich product catalogs," apps for resolving customer-service issues, and apps that make use of photos, videos, pen and voice notes, tied back in to an "asset database."
"Siena works well with corporate and web data and media content: SharePoint lists, Excel and Azure tables, RSS feeds and the gamut of RESTful services," Microsoft's app-description notes.
Internally, Project Siena may have been known as "AppMagic." (It's not clear if that will be the final name of the app, but a quick search makes it clear a few others seem to have claims on the names "AppMagic" and "App Magic.")