Django form validation vs DRF serializer validation
DRF's serializers are nicely integrated within the framework. Whether they are models based or not, you will have a nice browsable UI to play with for free and you can get OpenAPI schema generation. There are probably other things I did not think of.
What serializers can do that forms can't:

Nested data - although you can get some results with formsets.
Hyperlinked relational fields
Difference between an empty field and a blank field - HTML forms can't do that
out of the box integration with generic class based views or viewsets

Please note that your can still benefit from DRF even if you are not using serializers. DRF parts are loosely coupled.
I'm new to APIs creation in django.
I checked DRF serializers and I found them useful when manipulating models in DB. My APIs don't use models they are based on forms. 
So my question is : what can I get more from using DRF validators then using django forms validators in my views ? maybe the content type is more convenient  (application/x-www-form-urlencoded vs application/json) ?