What does the clause about seeing into the Ethereal Plane in True Seeing accomplish?

Question

True Seeing's text states:

For the duration, the creature has truesight, notices secret doors hidden by magic, and can see into the Ethereal Plane, all out to a range of 120 feet.

Here is the definition of truesight:

A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the monster can see into the Ethereal Plane within the same range.

(Emphasis added in both cases). Since truesight already includes seeing into the Ethereal Plane within range, and the range for both truesight and seeing into the Etheral Plane from True Seeing is 120 feet, what function does that clause in the True Seeing spell have?

Answer 1

They convey the same information in this secenario. You correctly interpret that True Seeing grants you a range of 120 feet., and that Truesight enables you to see into the Ethereal Plane within the specific range that is 120 feet.

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A common game design idea is to make information available that faces the person who needs it. The text of Truesight is GM facing and the text of True Seeing is player facing — see that Truesight appears in the BR chapter 12 part 4 Dungeon Master's Tools about the senses of Monsters. So while the information itself is redundant it accomplishes an emphasis that maybe isn't as obvious as the other aspects of what Truesight grants the player. Did the editor use that principle, or did they miss it? We don't know, and the editor probably doesn't remember anymore.

Answer 2

It's "helper text"

The D&D editors will sometimes repeat a rule where it's relevant, which is referred to as "helper text". It's not always actually helpful, though -- awkward or haphazard helper text can make a rule look like it means something other than what it really means. Jeremy Crawford has spoken in the past about this being a source of errata that doesn't actually change any rules, but clarifies things by striking the "unhelpful helper text".

Editing isn't simple or straightforward. If it were me, I'd have suggested that the true seeing spell ought to list off all the things Truesight does, then add the note about secret doors, which prevents the player from having to go check the index and look up a different page to find out what "truesight" means. But adding a few more lines of text to a page isn't an easy call to make in a book that's already overstuffed.

Actually, this might simply be an artifact of the development process. It's entirely possible that truesight did not originally allow ethereal vision, so that was in fact an add-on effect unique to the spell, but then truesight got edited to add the ethereal vision as a base effect and nobody noticed the spell text had become redundant.