What is the point of having a lot of line judges, since everything can be done automatically by Hawk-Eye?
I mean the microphone, that listens the ball touching the net, and Hawk-Eye, which knows where every ball lands during a point.
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First, Hawk-Eye technology is not available for every court that professional matches are played on. So, in those cases, players can't challenge a call because no Hawk-Eye is available. The chair umpire will inform players before they begin to warm-up if Hawk-Eye will be in use for their match or not.
Second, Hawk-Eye tracks the trajectory of each shot during a match, but it typically takes a little while (up to 10 seconds) for the chair umpire to tell the people in the Hawk-Eye booth to generate the video and show where the ball landed. It typically disrupts the "flow of the match" which is why players only receive 3 challenges per set (and an additional challenge during a tie-break). This also makes it so a player can't abuse the system and ask for Hawk-Eye review after many points.
Lastly - some points just don't require the use of Hawk-Eye technology. Sometimes the ball lands out by several feet and does not land close enough to a line to require a precise measurement of where the ball landed. In those cases, the line judges and chair umpire can easily make the call.
However, there are some fans of tennis (including myself) that feel that players should not need to challenge calls on shots that land close to a line, and it should just automatically be reviewed by Hawk-Eye. The ball can travel at very high speeds during a tennis match and getting the call right with the human eye can yield incorrect results - in those cases I think the line judges should be able to indicate they are unsure of the call and should just be able to rely on Hawk-Eye if it is available. I think the challenge system currently in use is still somewhat of a marketing gimmick and something to entertain the fans with, rather than being used for more precision in line calls.