Timeless Dragon is a beautiful Modernized version of Eternal Dragon from Modern Horizons 2.
Any deck that wants to run Eternal Dragon will most likely also want Timeless Dragon, since both provide the ability to smooth out mana early while providing advantage later. However, Timeless Dragon errs more towards the side of immediate mid-game advantage compared to Eternal Dragon's mana intensive loop.
Eternalizing a 4/4 flying for 4 mana is a powerful enough thing to do on turn 4 if you haven't found anything more impressive to do. It's also an efficient payoff if you've passively dumped it into the graveyard through some other means. The Eternalize ability can only be used once since the card exiles itself, but by then it has already provided more than a card's worth of value.