Hi,
Are you looking reason for why employee_id.user_id is used ?
In this case what they have done is that, for that user he can only view his own records, to achieve this the employee record and logged in user has to checked.
uid will hold the value of the current logged in user's id. Then it has to be compared with related user of the employee. All the employees is related to user, opening the employee form you can see a field named Related user.
The employee_id have the hr.employee record, from this record we can access the related user of an employee, so using the . operator, related user is accessed by employee_id.user_id and it is compared with uid, then for the records for which both matches will be shown.
If employee_id = uid , is given as filter , then employees id will get compared with the logged in users id.
Thanks