I think mechanics come with the territory. RPs need some semblance of them if the GM wishes to uphold a functional structure and reinforce the tone, after all, they're a type of game: not having an idea how you play into your own RP as a GM and improperly slapping your creation with the "sandbox" label because it's "free-form" (read: not fully fleshed out, "let's see what happens") is a mark of laziness. A lot of people think making up a world and saying "go" is all they need to run an RP, which is why a lot die almost instantaneously: the creator didn't include some way for the players to feel a sense progress or completion, or feel like they were affecting the world they existed in, or nobody could see where they were going with all this. Mechanics can help make the world feel like a living, breathing place and facilitate creative (yet fair) ways to impose challenges on your players, even if you genuinely would like to make no particular plot at all. Also, they keep muk grounded.
Mechanics are also vital if you wish to tell a certain plotline. I've said it before and I'll say it again: roleplays are not fanfiction, we have a lovely section run by a fabulous, attentive mod if you wish to write a story within an interpretation of a franchise or a world you've created. Because while having other players explore the world you create is fun and their character's tenacity/creativity might delightfully surprise you, you can't expect them to fall perfectly into place with your intended story, and you certainly don't want to forcefully drag them through it. Mechanics can be your best friend at finding natural ways to supplement parts of your plot in the lives of your player characters that makes them feel interested or excited to pursue them. A solid rule I've found is: people want to discover via actions they think up, feel special for performing something and getting plot-related consequences whether they intended to or not, feel like their GM is reading their posts and giving a damn about the work. Make mechanics that work with your players and that make your story fluid enough for them.
But as I've come to learn, you aren't able to to plot out every mechanic from the get-go. Sure, your baseline will probably not change, but at least in my experience (with the various RPs I've made over the years) is that certain trends with crop up that require you to streamline or expand upon what you've created, or will cause you to delete certain features all together. Be adaptable!
The basic point is, mechanics create necessary limits. While that word sounds dirty, limits have proven to breed consistent tones and brilliant solutions to problems, facilitate fun in believable ways, and make for interesting worlds: after all, aren't we all characters in our own limited world?
Hopefully someone gets something out of all this rambling.