On the other hand, TAI does not take into account the variations in Earth’s rotation speed, which determines the true length of a day. For this reason, UTC is constantly compared to UT1. Before the difference between the two scales reaches 0.9 seconds, a leap second is added to UTC.
On average, Earth has been slowing down a bit over the past decades, so UTC is currently running 37 seconds behind TAI.
Reasons.
Reasons just states difference, but does not answer why.
So leap seconds is advantage here?
There’s literally a section titled ‘why use UTC - not TAI?’.