what is need of nullable value types in c#?

The main reason for introducing them was to provide a better interface with database columns which can have 'no value' as well as a definite value.

When the SQL types correspond to value types in C# (int, double, DateTime etc), there was no satisfactory way to deal with this before nullable types were invented because a value type must have a definite value - it couldn't be null like reference types such as string.

Usually, some 'magic value' which either wouldn't (or was very unlikely to) arise in practice had to be used to denote the absence of a definite value.