self.ensure_one is made to ensure that only one record is being passed on. It checks that the current record is a singleton (so just one record, not multiple records). If self would contain multiple records the system would throw up an error. An example case to check you just have one record:
@api.multi def some_fuction(self): # This will make sure we have on record, not multiple records.
When you would run this code with a selection of records (for example selecting multiple records from a treeview) the self.ensure_one() would check this and throw up an error.
A general rule of thumb is to only use self.ensure_one when you are sure / want to be sure you only have one record, not multiple. In cases of you using multiple records in one action you should use @api.multi without self.ensure_one()
Have a look at the ORM documentation: https://www.odoo.com/documentation/8.0/reference/orm.html
To reply your second question,
You can use @api.one, but since @api.one put returned value in a list and it is not always supported by the web client, therefore most of the developers email@example.com with self.ensureone().
Refer to this link for more details,