To be one of the best Trainers, hatching egg is no doubt the most important thing as it allows you to fill up Pokedex, at the same time getting a higher chance for high IV pokemon. Now, there is even a bigger purpose for it, which is to hatch the Gen II baby pokemon, such as Pichu and Togepi.
One of the biggest concern would be: Can we hatch a Gen II pokemon with the eggs we incubated before the update? The answer is unfortunately NO. The pokemon that you are going to hatch is very much determined at the time, and the spot you get it. Trainers on Reddit had suggested that the Pokemon you hatched is very much determined by the Biome of the pokestop you got the egg. For example, when you get an 10KM egg at a pokestop near water Biome, you will get a bigger chance of getting a Lapras, Kabuto or Omanyte. It actually makes a lot of sense as you aren’t supposed to get a “fish egg” with all the rocks and mountain surrounded right?
Below would be my personal experience: I am one of the players who start playing PokemonGo since it had released. I have been consistently hatching Eggs and now I have hatched over 1200 eggs. Many players(me as well)complained about not getting a Lapras even they have been aggressively hatching eggs. However, during the Halloween Event, there is a much higher hatch rate of Lapras reported by players Worldwide.