private constructor, subclassing and sealed

Question

If one can prevent subclassing by declaring private constructor in the base class, why do we need "sealed" keyword? Is it so because CLI can optimize it better? maybe.

Thanks.

Answer 1

Because you might want to have public constructors but not allow anyone to derive from your class

Answer 2

If a class is sealed some optimizations can be performed. i.e. the clr could emit .call instruction rather than a .callvirt