There are actually a few companies making things like this, I first ran across them about 3 years ago. most of the ones I saw were little watch sized discs or boxes that were magnetically attached to a diaper and could be removed and used again on the next one.
They could actually be quite useful in the right circumstances, for babies and very young children not so much. They should be changed and checked regularly and it's easy enough to do, a device like this will just make parents lazier and possibly cause more harm than good.
But for instance their use in care homes or elderly residential homes where staff are already quite heavily overburdened, much of the time the residents can tell you if they need it, but having an alert system to say that a resident needs changing would help with the more stubborn or non verbal residents and save staff a little bit of time they can use for other important jobs. The same goes for hospitals with long stay patients that may be unable to communicate or move to activate a buzzer.
There was also a paper a while back about a device like this that could detect not just when a diaper was wet but how wet it was, a little device like this could be good for cloth diaper users as unless you are paying full attention (and there are some that can't tell when they've been without physically checking) its often difficult to determine how wet a cloth diaper is.
Again while they should be changed regularly, for an adult it is more down to the individual how long they want to take and knowing that your cloth diaper is only at 10% capacity or reaching 90% full could be handy.