In no schema db you don't have fixed columns and types.
For example product 'Jeans' can have attributes 'price', 'length' and 'model' (M/W) but for product book you have attributes 'price', 'authors' and 'title'. For mobile phones you will have 'screen type', 'operating system' etc.
It is very difficult to model that in RDBMS because you are not flexible and user cannot insert arbitrary attributes so it is easier to use a document database which are optimized for this kind of data so that you can easily search and filter by value on arbitrary attributes (eg. all products with length>30 and model=w).