well, i'm not an expert, but according to me that i read from many posts.
incubator is a *device* where you put your egg for 21 day by turning them till 18 day and bumping the humidity at 19-21 day.
hatcher is a *device* where you put your egg when it reached 19 day and have no turner and no need turning anymore also with high humidity provided, the eggs are moved from bator that only hold them for 18 days.
conclusion: you can hatch the eggs using incubator itself for 21 day full or you can keep the egg inside incubator for 18 days and move them inside hatcher for 3 last days.
the purpose of why people use hatcher is to have a fully running bator so even if you put 1 egg everyday to the bator you won't be afraid of *distortion* date of bumping humidity since you can mark the egg till and move the marked egg to hatcher when it reach the 19 and still having fully running bator.
the hatcher have high humidity and no turner.
the incubator have low humidity for 1-18 and high humidiy for 19-21 and with/without turner.
hatcher is also an incubator but all is depend of people how they want the bator and hatcher work.