The sole purpose of a profile in salesforce is mentioned below:-
Gives user access to Objects with permission to perform the CRUD operation
Read - this is always the first and minimum permission to ensure a user can view a record of say an Account Object
Create - Only when this permission is provided user can insert a new record in the Object
Edit - to perform any kind of data modification on a record or a group of record is done with this permission
Delete - permission to delete a record
View All -this permission when provided to the user can bypass all kinds of record sharing (we will discuss record sharing later )provisions in the applications, but the user can't make any changes to the records he is viewing
Modify All - this is the god mode and exists on its own , this would provide the user to perform any kind of CRUD operation on all records for an object
Profiles are a must for user creation, a user won't exist without a profile
Profiles don't do much with respect to record visibility which is enabled via sharing settings
More points will be added as more observations are uncovered.