I have seen many exchanges have launched their own token like Binance with BNB, Idex with IDEX, Hotbit with HTB, Yobit with YO etc. where exchanges use their own tokens as another way to pay fees to support token. In addition to it, different strategies are used to create a hype like decrease in supply etc.
According to me exchanges should promote the usage of their coins/tokens outside the exchange to build a community in support of it. If your community consists of huge individual entities who believe you then a great community can be formed. Otherwise lack of applications would lead these things to disaster.
Almost all exchanges tokens are only created to be used within the exchange, for lower TX fees, and other goodies, like the way only BNB holders participate in binance launch pad, and yo holders participate in any IEO on Yobit exchange, most exchanges don't really bother to go further than that, I remember earlier this year when someone asked yobits telegram moderator if there were plans to list their yo token on other exchanges, they replied "No", so that means exchanges don't really bother much about been on other exchanges, but I'm glad binance team has created a standard for their BNB token in or outside their exchange
But if an exchange goes through a hard period of time and shuts downs then ultimately that token will come to its end because there is no community other than that exchange supporting the token simply because it has no usage outside the exchange.
This is one of the reasons why investing or holding onto exchanges tokens are risky,because so many of them outside their exchange they are nothing, they only prove to be vital when used on their exchange, no strong community to back their token if their exchange eventually close down.