What might be the advantage of using such a "roundabout" route to get to the seventh rank?
Using a "Zwischenzug" can be very beneficial. It is also easy to accidentally blunder. However, this is the primary method of introducing chaos into the game. Usually the option to change the course of exchange (or the offering of) is chosen when a player realizes that they can gain an advantage by making a move which was unusual. Perhaps by issuing a check through taking a pawn by sacrificing a knight, and then gaining a knight from some discovered attack. The net result is that you won a pawn. These happen very often, and are sometimes either overlooked, or accidentally made available.
You should always be on the lookout for a Zwischenzug. They can wreak havoc on either your opponent, or yourself, and should be monitored closely.
You make a "throwaway" move that forces a particular response.
This is quite a different question. Often these types of moves aren't made until the end game, when moving a pawn can dramatically weak a position - or when moving the king leaves a pawn unguarded. These moves can also be made as waiting moves, where a trap is set, and the player does not want to unravel it so they will play a waiting move, usually along the lines of pushing an outside pawn; perhaps something like
a5. Timing these moves can be very important, but they are more eloquent than the more combative style of Zwischenzug.