I know there are some out there who legitimately believe in the witchcraft they're performing, spells they cast, crystals, etc., but for me, I do practice wicca, but it's less about thinking that I'm having a physical effect on the world and more of a way to almost meditate and for sure a way to connect to my spiritual side.
Even though I know that me performing a ritual to bring a friend some good luck or something won't actually do anything in the real world, it puts me in a space where I can reflect on my friend and what they mean to me, for example.
It certainly can be a bit silly, but many of us practice because we like the state of being and state of mind it can put us into, vs truly believing we're performing magic.
Spiritual practices often involve beliefs about the world that might not be compatible with scientific rationalism, and that's a valid criticism, but I think it's a mistake to only consider them as epistemological perspectives. The value of that stuff is for people to have tools for organizing meaning in their lives, conceptual foundations for community, culture, art, etc. I'm an atheist and have many of the usual atheist objections to religious beliefs, but spiritual feelings and experiences are valid and can be considered as distinct from questions about what is reality and how to accomplish goals, and I can respect people who are trying to explore those things.
I just saw a flyer stuck to a light post yesterday for a Witches Festival in my area next week. I took down the information and I'm planning on going! I mean why not? It seems like a fun and silly way to spend the evening and probably meet some wacky people that I thoroughly enjoy being around.