This is always awkward to answer questions like that. What would you mean by "we need"? Who are "we"? I always feel that if you are asking, you might not be the one who needs them… :-) See also my comment to the question.
You should be able to get an idea from these articles:
http://msdn.microsoft.com/en-us/library/ms731082.aspx[
^],
http://msdn.microsoft.com/en-us/library/aa738737.aspx[
^].
In brief, .NET introduces new, fully object-oriented platform; and in view of strictness and clarity of this platform, previously existing RPC, SOA or REST models with previously existing Web service standards look ugly, overwhelmed by system compatibility, legacy and old standard support issues, suffering from the lack of flexibility, clarity, from too much of "accidental complexity".
See also:
http://en.wikipedia.org/wiki/Web_service#Criticisms[
^].
—SA