Why do the Toa Mahri have Breathing Gear?
So, when the Toa Inika got into the pit they got mutated into the Toa Mahri by the pit mutagen, and they were only able to breathe in water. After they finished their mission, they got cured and were able to live on land again, although they still looked like Toa Mahri. But there is something really strange about this... Why do they have Air Breathing Apparatus if they can't breathe air but they can only breathe in water? Why all those cables and air tanks if they couldn't even breathe air?
Why do they have Air Breathing Apparatus if they can't breathe air but they can only breathe in water? Why all those cables and air tanks if they couldn't even breathe air?
Mostly because set design comes before story, as Greg often mentioned in his discussions about why certain things look the way they do (case in point, Jaller Inika's Energized Flame Sword, which he canonically possessed two of, as Greg was writing his novels based off of a prototype set that possessed two). The set designers are basically told, "OK, so this year we have an underwater theme," and they design the sets based on that. They don't know the exact circumstances behind the Mahri's ability to breathe water, so they go with the most obvious design aesthetic that can be carried over across all six sets, breathing tubes.
Additionally, story details are always malleable, so it's possible that at one point in the discussion, the Mahri would have been given underwater equipment. While this got translated to the sets, the story idea might have changed, resulting in the disparity we see in the finished products.
It's just a matter of sets.
They were transformed by the Mask of Life, not the Mutigen.
To repeat what others have said: because it looks cool and gives the theme of being underwater.
Sarda, Idris and Lesovikk also possessed the tubes, and were mutated by the pit mutagen.
Personally I like to imagine the tubes acting as gills of some kind.
As others have said, the scuba gear was clearly the idea of the set designers (to fit with the underwater theme), but Greg, or maybe the story team as a whole, wanted the Mahri to be water-breathers.
Greg stated at one point that the tubes are actually their gills. No idea how that works. Also, now that they can breath air again, apparently the tube gills are still present but are "just ornamentation now."
Well, he was officially talking about the Mahri.
Right, but it stands to reason they would serve the same function.
Agreed, but they do have different origins.
Presumably, they possessed the tubes before they were mutated.
Doubtful? They don't allow Sarda or Idris to breath water, as they still needed air bubbles. and neither Dekar or Defilak (who are not mutated) possessed them. Why would they in that case have tubes?
If that's the case, why do the Toa and mutated Matoran have them and the Barraki don't?
Simple, because Barraki are of different species, they're not Matoran. The mutagen affect different species in different ways. For example, the Skakdi turned into snake-like creatures, while the Makuta only had minor mutations and some of their abilities locked.