How can depositions help or hurt my case?
Depositions can be very helpful to your case because they will let you know ahead of time what testimony you can expect from the other party and from their witnesses. That way, you can better prepare yourself to ask cross-examination questions or enter documents at trial that might show that the other party or their witnesses are lying. You can also use the deposition transcript itself as evidence if the person who is testifying at trial says something different from what s/he said during depositions.
This also works the other way around though. Depositions can help the other side to build their case against you. If you say something in a deposition that later turns out not to be true or that you describe differently when you testify at trial, it could seriously hurt your case at trial. The other lawyer could use any inconsistencies in your deposition to call the truthfulness of your entire testimony into question.