Schema file is used for defining your database schema for your application. Schema file will generate all the required data structure for you. This schema provides backbone for generating model to Symfony.
Schema.xml file allows database specific content declaration for your table. It also allows complex database declaration for your schema. Mysql provides enum datatype but you cannot add this type in schema.yml file but you can add this in schema.xml file. This is just an example.
Here are a few problems also. When I tried to use enum in schema.xml file it did not give me any error but created trouble somewhere in model. So I think there is bug in this part of symfony.