Well, as far as my knowledge is, over-the-counter trade is for those who want to make large-scale investments in cryptos. These might also be for ICOs as the investment in cryptos needs to be converted into fiat currency for development of the project.
I don't know if such a kind of trading is of any benefit for small investors or individuals who want to convert their fiat into cryptos because bulk trading of cryptos might save a lot of cost for the traders!