The Express Pass allows for skipping the regular lines at the rides. You save a lot of time and get to go on many more rides because your wait times are anywhere from 50% to 90% lower than without. If you are going to multiple park the Express Pass is the only way to get the most out of your adventure.
To add to the previous answer, they are Express Pass Unlimited passes. They will allow you to use the Express Lane as many times as you'd like for any ride. The regular Express Pass only allows that once per ride/attraction.
If you have a universal ticket during your stay rhe express pass will get you access to the priority lines to get on the attractions quicker than the regular lines
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