There is a possibility that T4 does something beneficial that T3 doesn’t. But most likely the main benefit is just that T4 is like a reserve to pull from when it’s needed. T3 is fast acting and gets used up quick, and if it’s used up and the body needs more thyroid hormone, it’s beneficial to have a constant reserve to always be able to pull from, that the body can then convert to T3 as needed
It might be similar to how low SHBG men seem to have more problems thatn moderate to high SHBG men do. SHBG acts as a time release for the testosterone in ur system. So men that have a healthy supply of test for longer periods of time tend to do better than men with low SHBG levels and the test in their system doesn’t stick around as long and gets used up quicker.