There are 3 types of breeding mechanism:
Viviparous animals in which the egg is fertilised inside the female, and nourished via a connection to the mother (the placenta)
Oviparous animals in which the female deposits the fertilised egg somewhere and it is left to hatch on its own (if it isn't eaten)
Ovoviparous animals in which the eggs hatch within the mother, and the young develop inside the mother. Ovoviparous animals do not have a placenta to feed the young sharks, and they survive by eating whatever they can find - mainly unhatched eggs and each other.
All three of these reproduction strategies can be found in different species of shark. Oviparous sharks have to lay a large amount of eggs in order to ensure that a few of them will eventually hatch, and ovoviparous sharks also usually start out with large numbers of eggs, but give birth to one or two (the strongest). Viviparity has an evolutionary advantage, in that less eggs are produced and almost all of them eventually become little sharks.
As to the evolutionary reasons for such diversity, I don't think that anybody really knows. Bear in mind that sharks are among the oldest living creatures, so they have had plenty of time to try different strategies. It could be that viviparity is a relatively new breeding method on the evolutionary scale.