Noda Time exists for .NET for the same reason that Joda Time exists for Java: the built-in libraries for handling dates and times are inadequate. Both platforms provide far too few types to represent date and time values in a way which encourages the developer to really consider what kind of data they're dealing with... which in turn makes it hard to treat the data consistently.
While the .NET date/time types are actually easier to use than their Java counterparts (largely as a result of being immutable structs instead of mutable classes), they're actually less powerful - in .NET, there's no such concept of "a date and time in a specific time zone" for example.
Noda Time aims to change that with a library which is powerful and easy to use correctly. It is built on the underlying "engine" of Joda Time, but the public API has been largely rewritten, both to provide an API which is more idiomatic for .NET, and also to rectify some of the Joda Time decisions which the Noda Time team view as "unfortunate". (Some of these are simply due to having different goals; others I'd argue are really mistakes.)
For a more detailed critique on the problems with the existing
date/time support in .NET (and
DateTime in particular), see this
blog post. Of course, if a later version of .NET comes out with
a new date/time API, the Noda Time team would happily go into
retirement (other than for the sake of those forced to stick with
earlier versions of .NET, of course).