That is a great question,
People might use vanish to be unseen, for an example. Maybe people want to build by themselves and they don't want other players tp'ing to them or asking them "Hey, can you please help me with this"
I get a lot of messages saying "Can you please help me with this" from different and lots of people and I wouldn't waste my time doing /ignore. /vanish is pretty useful so you don't have to waste your time ignoring people. A friend might come on and ask if you can help them. Once your friend is online and if you know what they are gonna say, you can just do /vanish. I think its simple to see why people shouldn't pay gems for /vanish.
Otherwise,
If any of you disagree with me. I respect that