What does the value tag do in raw JSON text format?

As clearly stated by the Minecraft Wiki, which should be the first place you research to find this info:

value: Optional. If present, this value is displayed regardless of what the score would have been.

This feature is actually a remnant from older versions of the game. This value tag is what older score components were resolved to, instead of a text component. It's no longer used though. You can if you want to, but there isn't really a point anymore.

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P.S. Your syntax is wrong, the square brackets surrounding score and its stuff should be curly braces {} instead:

tellraw @p {"rawtext":[{"score":{"name":"name_ph","objective":"obj_ph","value":"value_ph"}}]}?