What is the need of Model validation as already we have UI validation

 What is the need of Model validation as already we have UI validation?

During my career initial days I'm thanking about this..Later I understood both are important. some of the validation's very easy to implement through model validation , some of them are very easy to implement through UI. In some cases both are important.

If we take API's, only model validation is the option because there is no UI. Applications which are consume API's , the only option is to UI  validation.

Some of the companies work on products those are need complex validation. Consuming that product as a custom controls