Group stages or round robin stages in a match play knockout event are used to ensure players play a minimum number of games before they are eliminated. Otherwise players that lose their first match would be eliminated after one game unless there is a plate or consolation event.
The main benefit of group stages is that it can enable players to advance to the knockout stages of a match play event even if they lose or tie a game.
This is used a lot in professional sport where sponsors do not want to risk top seeded or high profile players being knocked in a shock result in the early stages of an event.
The winner of the event does not have to win every game as they would in a normal match play event.
In the final round of the group stages there can be dead games where neither player can advance.
There can be uneven games where one player can advance to the knockout stages but the other player is already out. This means that the players are not playing under the same conditions which can affect the pressure and motivation each one experiences. This is quite prevalent in the group stages of the WGC Match Play golf tournament.
Group stages create the possibility of repeat matches if more than one player or team can advance from each group. Frequently two teams who have played each other in one group both advance to the knockout phase. They will be placed on opposite sides of the draw and then meet again in the final, often with the team that lost the group stage game winning the final.
This is a scenario that happens repeatedly in the rugby World Sevens Series where the same 16 teams play 10 events with 2 teams qualifying form each group for the knockout phase. However many other major events can suffer the same scenario such as FIFA World Cup Football, World Cup Rugby, UEFA Champions League Football and NFL Playoffs.
Each of the disadvantages can be avoided by using the Match Play Matrix format where every player plays every round.