Roster rules in American top-flight soccer are enforced against a budget charge, not against actual payroll. Allocation money is the mechanism that separates those two numbers.
The budget charge is what gets capped
Each club has a spending limit, and each player carries a charge against it. That charge is not necessarily the same as the salary the player actually receives.
Separating the two lets the league keep a firm cap while allowing clubs to pay more than the cap would otherwise permit for particular players.
Allocation money is the tool that lowers a player's charge. Applying it reduces what he counts against the limit while his compensation stays where it was negotiated.
Clubs acquire it in defined ways
Allocation money is not simply purchased. Clubs receive it through mechanisms the league defines, such as selling a player abroad, missing the playoffs or qualifying for continental competition.
It can also be traded between clubs, which is why American soccer transactions frequently move allocation money rather than transfer fees when two league clubs deal with each other.
Because it is finite and league-issued, it functions as an internal currency. A club's practical spending power is its cap space plus the allocation money it has accumulated.
It solves a specific roster problem
The designated player rule lets clubs exceed the cap for a small number of players, but the rest of the roster still has to fit inside the limit.
Squads therefore hit a squeeze in the middle, where a club wants to pay well for several useful starters but cannot fit all of their charges under the cap.
Allocation money is aimed exactly there. It buys down the charges of good non-designated players, which is the tier that determines whether a club has real depth.
The trading market has its own logic
Clubs value allocation money differently depending on where they are in a build. A rebuilding club with cap room may prefer to trade it away for players or draft position.
A club close to contention will pay well above face value to acquire it, because a small reduction in charges can be the difference between signing a starter and not.
This creates a genuine internal market. Front offices track their allocation balances the way other leagues track cap space, and it is a routine part of trade negotiations.
Complexity is the price of the system
The rules require considerable expertise to use well, and clubs that master them gain an advantage that has nothing to do with scouting or coaching.
Supporters find the system opaque, since a transaction's real meaning depends on charges and balances that are not always public.
The league has periodically simplified and renamed parts of the mechanism, but the underlying design persists because it is what allows a hard cap to coexist with a competitive player market.