For me, it's dedicated time focused on the subject area without operational distractions, a little more sticking your head out of your company and meeting other people with similar interests but much different contexts, and a chance to ask a question of experts in person who would otherwise never talk to you. You don't get these from attending an online conference in my experience.
Those things can shift strategy or tactics in ways that are a force multiplier so I consider it worth my or my teams time.