Noah definitely had his own reasons for saving Shisui and using Holy Light to restore his eyes.
Though Noah neither envied nor coveted the Uchiha's power, it was still a means of protecting himself. Especially for someone like Noah, whose taijutsu was practically useless, that thing called Susanoo was really quite appealing.
Without the protection of a magic shield, Noah had never felt truly safe. Otherwise, he would not have immediately considered finding an Uchiha body to possess back then.
His interest had gradually faded later on only because his research into and understanding of Holy Light had given him a terrifying physique, allowing him to slowly let go of that idea.
But now that he had encountered the opportunity directly, Noah naturally would not let it pass. While repairing Shisui's eyes, he also carefully experienced the differences of the Sharingan.
The result left Noah stunned. This thing did not seem all that remarkable?
Other than the abilities reflected by one's mind, which Noah could not quite make sense of, he found that these eyes were essentially just another manifestation of Chakra.
Even the abilities reflected by the mind after reaching the Mangekyō stage gave Noah a certain idea. It was probably like this: your Sharingan was a key without a defined shape, while all kinds of abilities were locked behind different doors.
Once your eyes reached the Mangekyō level, powerful mental stimulation would cause that shapeless key to automatically transform, according to your thoughts, into a suitable key and open the correct door, granting you the ability you most wanted—or rather, the ability best suited to what you currently desired.
The Chakra represented by the Uchiha's bursts of mental power was actually an extension of Yin Release, and this type of Yin Release power seemed... to be something Noah