There's a good thread on timings already on the boards:
http://smnetwork.org/forum/index.php/topic,4620.0.htmlThey are marked from the beginning of the score through the end- not marked from the cue itself. They can be used to help place lighting cues, paged warnings for entrances, prop handoffs, costume changes, etc. Basically, they let you know how much time a given section of music takes and you can use that information in whatever manner is most useful for you.
If you're creating a document like a WWW (who what where), the timings are crucial in placing all of that information accurately on the page. The music will take the same amount of time to play each night (with very little variation) and these timings can fairly accurately tell you where you are in the music.
Does that help? There's a lot more that can be said about timings but I think the above thread covers some of them. Lots more opera info on this thread:
http://smnetwork.org/forum/index.php/topic,1249.0.html