[{quoted}](name=Fondling Gems,realm=NA,application-id=3ErqAdtq,discussion-id=nALv9bwY,comment-id=0001,timestamp=2017-11-07T23:47:33.052+0000)
I think it is to facilitate her ability to gank other lanes. Mid laners are meant to be pretty good roamers, especially kill focused ones. Her kit points to it too with her E traveling further over walls and whatnot.
40 seconds tends to be a good time for roaming. It facilitates the "Bot lane flashed, come gank Zoe." while also putting a time limiter on her to get down there to make use of it. Generally it takes about 20 seconds to get to another lane without boots, considering Zoe doesn't appear to have a ton of waveclear early and it is based on what enemies use summoners, not up to her discretion 40 seconds seems about right. Unlike a good number of other roamers her mobility doesn't actually let her get there any faster either, unless she happened to pick up ghost or something.
I'd assume the bubble drops have a range necessary to them. Global minions randomly dropping powerful spells for a single character would be not just busted, but a cause for major screen clutter when it isn't relevant most of the time.
10 seconds would be plenty to make use of them during a skirmish.