I can't believe you never thought why did I bring the shirt tucking in the first place
Oh that’s right, I’ve been doing this for 14 years and have never once considered whether it was pointless 
Or perhaps I’ve been doing it for 14 years because I actually find it useful? It’s not about the shirt, I don’t give a toss about shirts. It’s about standing at the threshold of my classroom and saying ‘you’re not coming into my space until you meet my expectations’. It’s a routine and a reminder, creating a mindset. Do it enough at the start, and they’ll come up the stairs towards my classroom and tuck their shirt in without me saying a word. They know the rules, and they know I’m in charge of my room, which makes classroom discipline a damn sight easier once we’ve had an orderly entry (of which tucking in shirts is a part).
Or y’know, they could just all pile in chaotically?
As I said upthread, if a kid is going to stand at the door and refuse to tuck their shirt in, then you’re going to have problems inside the classroom too. It’s not the request to tuck the shirt in that’s the problem there.