When someone testifies in a court proceeding, a written transcript is created. It can be testimony given at a pre-trial hearing, called a deposition or examination before trial. It may also be from a different trial.
Lawyers often try to track down prior testimony of medical experts in order to see if the expert has testified in a similar case. They also look to see if they have testified differently than they are expected to testify in your case.
The purpose of tracking down prior testimony is to use during cross-examination.
First, you get the witness to commit to giving testimony in your case. Only if it contradicts testimony he gave in a prior case do you use the prior contradictory testimony.
 Get the witness to commit and lock them into their position at trial. you do not allow the witness to wiggle out of the testimony they have just given. Once they have fully committed,  you then pull out a prior transcript.
“Isn't it true that two years ago you gave testimony in the following case?”
"Isn't it true you were asked to testify as an expert in that case?"
"That case involved a failure to diagnose, just like in this case correct?"
 You snt the witness up with questions that locks him into his position as an expert who is testifying.
"On page 12, line 7, you were asked this question and you gave this answer, correct?"
"That testimony, given only two years ago, directly contradicts your testimony given in this case correct?"

There many ways to set up a witness when you have contradictory information. This is but one way to do it.