Generally OTC desks are useful for large purchases. Let's say you want to acquire $100 million in Bitcoin - you could go to Coinbase and just do it, but you'd drastically move the market, have a really obvious candlestick, and an observant third party could look for large movements from Coinbase and potentially track your holdings / de-anonymize you.
An OTC desk can be setup to spread the buy over several days / months, buy across markets, and optimize for best price execution. It's not necessarily a huge business, but a lot of businesses and high net worth individuals will use these types of services instead of trying to learn the skills themselves.
Not exchanges. The reason is that, as GP indicated - oftentimes execution is spread across venues.
This is typically a role of a dedicated broker house or there is a special area in many investment banks that takes care of that as part of customer flow handling.
So, I've read this, the announcement on XPrize, and a few articles, but I can't for the life of me figure out what the contest is, or what they want people to build...
Notice how that old link to http://www.xprize.org/ted now redirects to the IBM Watson prize. Which itself was vague. "this challenge requires an AI to form an opinion and support it with facts". I wouldn't be surprised if there's almost no link between the two.
An OTC desk can be setup to spread the buy over several days / months, buy across markets, and optimize for best price execution. It's not necessarily a huge business, but a lot of businesses and high net worth individuals will use these types of services instead of trying to learn the skills themselves.