its definitely not necessary, but its definitely helpful, yes some people prefer without but the vast majority do generally prefer a cam, it makes the videos more personable, humans prefer to see a person when they are talking, its only natural, you want to see the reactions of the player, makes your videos more about the person and not just the game you're playing or the video topic, meaning your viewers will watch more of your videos on different topics and not just the one they first came to watch = more views!
i introduced a webcam after about 3 months and never looked back, i would suggest to every gaming channel to have a cam, i would also suggest getting a basic green screen set up, its easy to do, cheap, makes the channel look a bit more professional + it takes up less space on the screen