Most of the times fields are nullable

In a typical type system in most programming languages the concept of null is being differentiated from common types:

• A common type: e.g. String.
• Nullable version of a common type:
  • E.g. String? in Kotlin.
  • string | null in TypeScript.

A common type does not include null by default, you want it? you need to explicitly add it.

But Unlike these type systems, GraphQL types are nullable by default unless you explicitly declare it as non-nullable using String!.

[!NOTE]

Each field being by default nullable is because there are many things that can go awry in a networked service backed by databases and other services.

[!IMPORTANT]

When designing a GraphQL schema, keep in mind when null is an appropriate value for a failed field, and if it is NOT, use non-null types.

[!CAUTION]

A common misunderstanding about GraphQL:

The nullable/not-nullable feature has nothing to do with user input validation and definitely should not be expected to do any of that. The nullabilty feature of GraphQL fields has to do with API integrity/ quality, versioning, etc.

— Ref.