Seems gimmicky and unnecessary. A stunt.
Why is that better?
I assume by "it" you're asking about using real time to affect in-game events, which I've done before and actually am planning on doing tonight, as opposed to asking about the interesting idea which I've never done before: tying game time 1:1 to calendar time.
Furthermore, I'll pretend you are asking why it's better in certain scenarios, as opposed to challenging me to prove that it's better in every scenario all the time, which would be a straw man.
The main reason I like real-time clocks is for the effect it has on player behavior.
1.) If you end a 5E game session when players "take a long rest", i.e. bed down for the night to recover spells and hit points, you reduce the incentive for so-called Five Minute Work Days even without having to change the game rules. (Hey, this is relevant to the discussion on proceduralism!)
It increases the chance that at least one player will say, "Hold on, I'm not done yet," and will press on, either splitting the party or dragging other PCs into greater danger with him. I don't typically see the Five Minute Work Day problem in practice so this isn't a big issue for me compared to:
2.) It helps ensure that we all get dramatic closure. Tonight I'm planning on running a mini-adventure one-shot to test out WotC's new Vecna stat block (I'm extremely skeptical about it), but I only have a couple of hours to do it. By setting a one-hour timer in full view of everybody, and secretly resolving that no matter what else happens, Vecna will arrive when the timer hits zero even if they're in the middle of another battle, I ensure that at least half of the time I have available for the game will be dedicated to the thing I meant
to do (testing Vecna) instead of incidental details.