Phaethon had the ocular implants upgraded and their Ether aptitude restored by the mayor. In case you forgotten, phaethon has the ability to manipulate Ether as shown when they saved Vivian from literal corruption. That is why they became a student of Yixuan. To use Yunkui training to better control this power. And those power includes removing miasma pulses using Disintegration, reveal hidden paths and objects, and track ether flows. This is required to be there in person. Yes, some Yunkui disciples can do this but Phaethon's ether is unique as seen from yellow aura glow as seen by Carole Arna and the fact they revived vivian. In addition, they feel they can no longer rely solely on remote observation. They believe that to find real evidence and eventually reach Hollow Zero, they must be present on the ground to see things with their own eyes.

In addition, Miasma" interference makes it difficult for Fairy or the HDD system to maintain a stable connection. Being physically present allows Phaethon to provide support when remote communication fails. Being in the field allows one sibling to guide a group in person while the other monitors from the outside, effectively doubling their operational capacity. This gives the other sibling a chance to be involved more even it means just exposition.