"Fast Track" Authority is a solution sought to a problem created by the division of authority
designed into the US Constitution
Under the constitution, the Executive branch, is charged with the negotiating of a treaty. The Congress is charged with the approval of the treaty. The Constitution is written that way because the Founding Fathers wanted a treaty to be acceptable to America as a whole, rather than a particular faction. By forcing the Executive and Congressional Branch to agree, they ensured that any treaty that bound America would enjoy wide popular support.
The problem came about when Congress, under its authority began amending treaties and sending them back to the Executive Branch for re-negotiation. This proved occasionally useful in winning further concessions, but mostly it incensed the people we negotiate with.
Think about it from a foreign point of view. You just spent two years negotiating a treaty, only to have it sent back amended. Essentially, this action is a unilateral demand for further concessions. Yet you sacrificed for the treaty that was just returned. Many countries are justifiably offended, and refuse to negotiate further, resulting in a diplomatic stalemate.
If a country expects this sort of Congressional action, they change their negotiating strategy,. They stonewall much earlier than they otherwise would, because they need to hold something back for the treaty's expected, amended, return. US negotiators encounter greater difficulties in negotiating, and often have to offer more concessions than they would like. Many possible agreements do not happen for this reason.
"Fast Track" authority sidesteps the process, and represents a compromise. Congress gets to approve a treaty, but not to nitpick it. As all treaties represeent compromise, all treaties are vulnerable to criticism. Congress gets to choose, is this good for us overall, or is it bad. But their sacrifice enhances the position of US diplomacy. Our envoys are seen as legitimate negotiating partners, capable of speaking for their country. This makes it easier for a rival to compromise.