Here is how a parlay works.
A Parlay basically consists of multiple bets combined into 1. This way you get a higher payout if everything wins, however even if one of the bet loses you lose everything.
So now if you have 3 bets , Bet A paying of 2x on winning, Bet B paying of 2x on winning, Bet C paying of 2x on winning. Now if you add all 3 bets in a toto or parlay, then if you were 2 bet 2$ on the parlay, then your winning will be (2$)*2*2*2. Its just combining those odds.
But if anyone of those games was to lose, you will lose everything. In simple terms, it compounds all those bets.
Unfortunately, there isn't too much of an advantage of doing this rather than those three bets independent upon each other if you're not looking for high risk [or to continuously wager until the end]
(Though if you are then it is the reason why Parlay exists)
Because of this, (I don't like to go completely all-in consecutively) I choose to play it safe without using the system.
I agree that there isn't much of an advantage. Honestly I don't believe in the odds increasing part, and I only make parlays with bets I am confident of. I use it more as a tool for games which start about the same time, when I can't get the money that I have to both games at the same time.