The transition volley is effective on the opponent's reaching backhand
This play works particularly well on an opponent's backhand side, especially if the shot is a two-hander. When stretched wide and off-balance, a player will often be forced to hit with one hand, a physically weaker shot, and one that generally necessitates a defensive, sliced return.
But even players with strong, one-handed backhands will resort to the defensive slice if forced severely. And with the ball moving slower and higher over the net (in an attempt to maintain depth), the best percentage play for hitting a winner is to take the ball in the air.
This reduces the opponent's recovery time and puts one in position for an easy kill, even if the first volley is not conclusive. The alternative is to allow the ball to sail back to the baseline, giving the opponent more time to get back into the court, and to go for a winner with a groundstroke.
Because of time and distance considerations, this shot must be hit very hard and close to the lines, imposing greater risk of error.
The threat of the volley produces errors
A secondary benefit of the transition volley is that it will cause one's opponent to make more errors.
Opponents get jumpy when surprise volleys preclude them from hitting low-risk, defensive returns and regaining proper court position. They are now forced to hit more severe and perilous shots from awkward positions.
This factor also operates as a result of the occasional serve and volley. They no longer dare to hit the soft, deep, chip return off the big serve, lest it be intercepted in the air.
Now they have to hit with less margin against a fast-moving ball that is difficult to control in the best of circumstances. The result: more serve return errors.
Today's players don't chip and charge
One might question why Federer and the others have opted to give up on the old serve-and-volley, chip-and-charge type of volley and replace it with the new transition volley.
The answer, of course, lies with today's more powerful and accurate groundstrokes, heavier balls and slower courts.
In order to profitably attack at the net these days, one must come forward behind heavier artillery than in days of yore. (In fact, in the old days of sliced backhands, fast courts, and light balls, one almost had an engraved invitation to come to net).
Now the serve returns and passing shots are hit too hard and too accurately for players to venture forward other than behind substantial heat.
If today's top players are allowed to remain on-balance and given time to set up and hit passing shots, the odds appear to be against even the most proficient volleyers, with the possible exception of matches played on grass.
Tennis evolution
Tennis is constantly evolving as players come up with bright ideas in response to new tactics and techniques. Tennis, in its early years, was a game played primarily from the baseline with relatively flat or sliced groundstrokes.
In the 1940's, Jack Kramer discovered that the persistent volleyer had the advantage on fast courts against this type of player. Hence, the serve and volleyers dominated the game for the next 25 years.
To counter the volleyers, players developed topspin ground-strokes while, at the same time, the courts were slowed down and the balls made heavier.
By the mid-1970's, Borg, Connors, Vilas, Solomon, Dibbs, and the rest killed off most of the volleyers. The game was dominated then by baseliners who won largely by attrition, with the exception of Connors, who actually provided a preview of today's transition volley strategy.
The next major development, led by Ivan Lendl, was the aggressive baseliner, who won by attacking from the baseline. And that has been the trend up until today, with most players winning as aggressive baseliners.
Of course there were plenty of exceptions to these trends. John McEnroe and Stephan Edberg were serve and volleyers during the aggressive baseliner phase; Dick Savitt won Wimbledon in 1951 as an aggressive baseliner in the days of the serve and volleyers; and Maurice McLoughlin won by serving and volleying in the 1920's when the early baseliners ruled.
But these people were exceptional in their times, and we are discussing general trends. And the newest general trend appears to be that of the aggressive baseliner additionally armed with the transition volley.