std::marker::Sync : a type is safe to use in a shared ownership model across threads
std::marker::Send : a type is safe to cross thread boundaries
A Sync only type makes no sense. You have to first move the object across thread boundaries to make it usable in a "true" shared context. You cannot share plain old references &T/&mut T across thread boundaries because the lifetimes (other than 'static) come into play.
What use cases will get covered by having a Sync only type ?
Sync trait seems to be of no value without a Send . Send is what enables sharing so why was Sync trait created ?
I understand the argument the Sync is of limited value without Send. E. g. using Arc will require a Send + Sync bound.
As @kpreid pointed out, with scoped threads (or similar API, such as rayon's join function), you can actually send non-static things between threads, so non-static references to Sync-only types can be used there.
However even if that wasn't the case, I cannot follow your argument why Sync should be useless. Even if Sync without Send wouldn't be useful, you'd still be distinguishing Send types (i. e. Send + !Sync ones) from Send + Sync types!!!
Like.. it's quite simple: if you share something, e. g. via Arc, you often need Send + Sync (though as mentioned above sometimes only Sync can be sufficient, too) and if you only need to send something, e. g. through a channel, then you only need Send. (Or if you share some data contained in a Mutex, then Send alone is sufficient, too.) Sync + Send clearly enables more things than just Send, and also it's implemented by fewer types. (E. g. a RefCell<T> or Cell<T> is not threadsafe when shared, but it's still Send [whenever T is Send].)
I have just fished the original version of this question as it had been posted to URLO out of our spam filter that had unfortunately caught it. It is getting engagement in both places now, so in case anyone wants to read all the existing answers or write new answers (especially longer answers with more effort spent) it may be valuable to take a look into the other thread as well to be aware of what was already said