What is the purpose of modes of discovery?

The Rules of Court provide six (6) different modes of discovery that parties may make use of. They are under Rules 23 to 28. They are the following:

[1] Depositions pending action (Rule 23)
[2] Depositions before action or pending appeal (Rule 24)
[3] Interrogatories to parties (Rule 25)
[4] Admission by adverse party (Rule 26)
[5] Production or inspection of documents and things (Rule 27)
[6] Physical and mental examination of persons (Rule 28)
The various modes or instruments of discovery are meant to serve as:

[1] A device, along with pre-trial under Rule 18, to narrow down and clarify the actual issues between the parties, which are the real reasons why they are in court; and
[2] A device for ascertaining the facts relative to those issues.

Such device exists to enable the parties, subject to rules on privileges, to obtain the fullest possible knowledge of the issues and facts before civil trials and thus prevent that said trials be carried on with the parties groping in the dark.