Shido's calling card. It really gives you the feel that yeah, he's the final baddie you gotta take down. (Granted he isn't but still). Everyone else thus far, a pervert teacher (Lust), a plagiarizing artist (Vanity), a crime boss (Gluttony), a careless company president (Greed), a corrupt prosecutor (Envy), and a depressed girl (Wrath) - all of them only had a small sphere of influence, relatively speaking - but Shido's different. A simple, physical card won't do anything to wreck him, the head of Pride. C.S. Lewis once wrote that pride is the root from which all other deadly sins grow from, and each of the prior Palace Rulers had a connection to him, direct or indirect. So of course, the Phantom Thieves have to truly go over the top to get him to react considering he believes himself to be untargetable (even seeing the thieves as nothing but tiny pesky rats), with all of Japan backing him. The only way to prove him wrong is to show all of Japan what a scummy ******* he is.
Each thief gets a line and plays a part in revealing how much of a ****ty person he is, and that final moment, where out beloved silent protagonist finally speaks...
"Yes, and before that happens, we will take [AKA Steal the Heart of] this country!"
It's just like ecstasy.
Then when we enter Shido's palace and hear "Ark" for the first time, just... wow.