A lot, I guess. Famously, Java's String worked that way (sharing the buffer for sub-strings and only storing offset and length) for a long time, until they changed that, I think in Java 7 to the same what .NET uses currently (copying the sub-string).

Both approaches have benefits and drawbacks, but apparently copying the sub-string seems to be best for the vast majority of cases and you only benefit from the shared buffer in select cases (also it's a great memory leak opportunity if you don't know how it's implemented). So I guess it makes sense for .NET to add the capability (in a more general form that also works for other things) instead of only having one or the other.