Incubators are needed to hatch a Pokemon egg. There are 2 kinds of incubators in Pokemon Go.
You can get the following 3 things by hatching an egg.
You do not know what Pokemon you will get from the Pokemon Egg up to finishing hatching. Pokemon from each egg is assigned CP randomly.
The amount of Stardust and Candy you can get by hatching Pokemon egg is much larger than that by capturing wild Pokemon. So, incubating and hatching are important for getting those items.
The speed limit for hatching egg is 24km/h to prevent cheating such as driving a car. To earn hatch an egg faster, you need to do walking, running, or riding a bicycle.
Since you can hatch 3 eggs per limited-use incubator, you should place 10kem eggs in such incubators. That means the limited-use incubator works for 30km.
There are 3 kinds of eggs: 2km (1.25 miles), 5km (3.1 miles), and 10km (6.3 miles). The longer the distance is, the rarer Pokemon shows up. Here is Pokemon hatching chart.
Best Pokemon to defeat Gyarados
Best Pokemon to beat Snorlax
Advanced Tips for Catching Pokemon
Charmander nests and spawn locations
The best moveset for Charizard
Best Pokemon to fight against Venusaur
What is DPS and EPS in Pokemon Go?