The main use cases are actually way more mundane, but also much more impactful. Consider some of the most common string manipulation code you see: grab a substring, compare it to some stuff, branch based on results.

Since strings are immutable and non-sharing, all substring calls will create copies of the substring. With Span<T>, you could instead simply request a span of the string and, with unification in the underlying typing, you can perform all of your string manipulation with no allocations or copying.