Whether it's 'hydras' or something similar, the single thing that got me into properly using Emacs was this feature in Spacemacs. For almost any situation, I can either pull up the spacebar 'global' menu, or a context-specific quick-menu. Discoverability has been insanely good, and I keep finding new tricks just by exploring the various menus that pop up.
I wish 'UI autocomplete' like this was a standard feature of operating systems. GUI's aside, how nice would it be to run a CLI command with some flag that allows you to explore the various options and string them together, rather than reading the manpage and searching for whatever flag you might want to use?
They're a big part of what make Emacs useable for me.
I have set up hydras for pretty much every major mode I use (except magit, which kind of has its own hydra-like menu system).
Because of hydras, I don't have to remember all the arcane keyboard shortcuts these modes have bound by default, and don't have to look up how to run a function that's not bound at all.
All I have to remember is one keyboard shortcut which I have configured to bring up a hydra in every mode, and it'll show me a menu of everything I can do in that mode.
This is really fantastic, has revolutionized the way I use Emacs, and has made it 10 times as useable for me as it was before I used hydra.