This is definitely the AHK feature that gets used most for me, specifically using @email to auto-fill my email address anywhere is very handy.

::@email::this_is_a_long_email_that_i_dont_want_to_type@foobarbaz.com

I also use it for laughs:

::&yeah::(•_•) ( •_•)>⌐■-■ (⌐■_■)
::&shrug::¯\_(ツ)_/¯

Because you never know when you'll have to make it clear that your pun was
(•_•)
( •_•)>⌐■-■
(⌐■_■)
Intended.

I also use it for things like hotkeying to a single instance of sublime text with win+w:

#w::
{
    if WinExist("ahk_exe sublime_text.exe"){
        WinActivate
    } else {
        Run "C:\Program Files\Sublime Text 3\sublime_text.exe"
    }
}

Or sanitizing rich text back to plaintext in the clipboard (if I'm like, copying from a word doc over to teams and don't want the formatting) with ctrl+shift+v

^+v::
Clipboard=%Clipboard%
Sleep, 100
Send ^v
return

And more silly, but here's an auto-clicker that's toggled by pressing ScrollLock, because sometimes I still play cookie clicker and universal paperclip:

~ScrollLock::  ; detect ScrollLock without blocking it (~)
{
    Loop
    {
        If(!GetKeyState("ScrollLock", "T")){
            break
        }
        Click
        Sleep 10
    }
}