Why does the QI role exist?
The key element in a successful exchange is that the taxpayer is not, at any time, in receipt of any portion of the funds from the sale of the relinquished property, otherwise, the exchange fails. Prior to 1984, the buyer of the relinquished property would hold on to the funds until the seller found a replacement property.
If for any reason, whether it was bankruptcy or judgments or simply buyer's remorse, the buyer did not have access to the funds at the time the seller needed them, it created a very large problem for the seller to be able to complete their exchange.
As a result, a safe harbor for the taxpayer was created within the regulations, and the role of the qualified intermediary was born. Now, instead of transacting directly with one another directly, all parties buy and sell via the QI.