Seeding decides which players are kept apart in a draw. It exists to protect the shape of a tournament rather than to advantage anyone in particular.
The problem with a random draw
Drawn purely at random, the two strongest entrants could meet in the opening round, and one would be eliminated before the event had properly begun.
That outcome damages the tournament, because the later rounds are the ones spectators attend and broadcasters pay for.
Seeding places the strongest players in separate sections so that, if results follow form, they can only meet at the stage the draw intends.
How the placement works
The top two seeds are placed at opposite ends of the draw and can only meet in the final, and the next pair are distributed into the remaining quarters.
Lower seeds are allocated to sections in pairs, often with a random element within each pair, so that the exact matchups are not fully predetermined.
Unseeded entrants are then drawn at random into the remaining places, which is where the genuine uncertainty in a draw sits.
Ranking is not the only input
Most events seed strictly by ranking, but some adjust for surface, on the reasoning that a ranking earned largely on one surface overstates a player's chances on another.
Such adjustments are contentious because they replace an objective list with a judgement, and players who lose places tend to object publicly.
The counterargument is that the purpose of seeding is to predict results at this event, and a surface-blind ranking predicts them less accurately.
What seeding does not guarantee
A seed still has to win, and the protection extends only to who is placed where, not to any advantage during a match.
Because the field beneath the seeds varies enormously in strength, two players with adjacent seedings can face very different routes to the same round.
This is why the draw ceremony attracts attention. The allocation of unseeded players materially changes each seed's prospects before anyone has played.
Why the number of seeds changed
The size of the seeded group has been adjusted over time, with larger fields at some events protecting more players from meeting each other early.
A larger seeded group reduces upsets in the opening rounds but also reduces the chance of the unexpected results that give a tournament its interest.
Organisers therefore balance predictability against jeopardy, and the number they settle on is a statement about which of the two they value.
