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.