I’ve been to several. It’s always nice to provide support to total strangers, just by your presence, and to contribute your prayers to their own for the deceased. Usually, the people who are present, but who didn’t know the family/friends/deceased will sit towards the back, though there’s no particular rule that segregates you from everyone else.
Once, I was in a city about 45 minutes away from my home, running errands. I randomly decided to pop in the church for their noon Mass. We realized there was a funeral about to start. I almost didn’t go in, because I hadn’t been expecting to go to church in the first place, and I was wearing my scruffy jeans and T-shirt for hauling the lumber and building supplies I was planning on picking up. But I decided to go in anyways and add my prayers for whoever had passed away. One of the church ladies from my town was there in the vestibule and saw me. “Oh, I’m so glad you could make it!” It turns out, the funeral was for her twin unborn grandbabies. It was one of the most moving funerals I’d ever been to, hearing what those little guys had accomplished even without ever having been born— and I was happy that I had chosen to go, rather than turning away because (it was a stranger) (I wasn’t dressed correctly) (etc).