Start and Finish fields is often called as the current/schedule start and finish.
In Project 2007 and earlier versions, these fields are adjusted based on calendar, predecessor and constraints. These fields are date fields.
In Project 2010 and later version, Microsoft introduced the Task Mode. This special field can be set to Manually Scheduled or Auto Scheduled.
By default, new tasks are created as Manually Scheduled tasks. This means their Duration, Start, Finish will be empty. This implies Start and Finish fields are text fields and not date fields.
When you link this task to another task, Project will default the duration to 1 day?, and calculate the Start and Finish field, but this will happen only once. Are you wondering why does this done only once? In File, Options, Schedule tab, by default "Update Manually Scheduled tasks when editing links" is checked.
I have written another post "The Little Benefits of Manually Scheduled Tasks". As mentioned in this post, Project draws a green wavy or red wavy line below the Finish date to tell the task needs to finish early or needs to finish late.
Without changing the task mode, how to see the recommended start and finish of the manually scheduled task?
Scheduled Start and Scheduled Finish fields displays the recommended Start and Finish as calculated by Project based on dependencies, constraints and calendars. This field can be used in custom formula and conditions.