Thanks for sharing yourself. Exactly. "Where the employer acknowledges that the employee is relieved" or is not an employee of them with an effective date and free for other employments. (A relieving letter is important for both Employee and Employer.)
Importance of Relieving Letters
In other words, for the Employer, a relieving letter is proof of separation and that they have relieved the employee from services completely and are not liable to pay any more in any regard. It also serves as the acknowledgment of the employee for them with that special purpose for their future reference.
And for the Employee, as you shared above, it is also proof of separation that they have worked with an organization for a period and are now free to seek other employment.
Notice Period and Relieving Letters
To address the situation where an employee who has already resigned but is still under the notice period, they will still be an employee and deserve the salary for that period until the last day of the notice period. Therefore, they can't be offered an experience/relieving letter. It is also self-explanatory "Relieving Letter" and can only be issued after getting "Exit Clearance" done.
Value of Relieving Letters for Future Employers
Regarding the second question and the value of it for future employers and companies, and why they require it from candidates (Future Employees), it is to verify whether they are relieved from their past employer or not, or to prevent unwanted future threats such as dual employment problems.
Hope you got the answer to your first questions in detail.
Also, I would like to know about your above statement, please clarify this.