Animals that lay eggs do not have to consume as many or as much food. They also never become awkwardly round before giving birth. The downside is that they have to incubate and guard the eggs and balance doing that with gathering food. But when the eggs do hatch, (in most species (not songbirds)) the young are either able to fend for themselves (crocodiles,frogs,fish,snakes,) or are able to keep up with their mother as she resumes her daily life almost immediately(ducklings). The oviparous animals can not move their young to a new spot if the mother feels it is not safe, not before they hatch or after.
 
The young of viviparous animals are either fully and completely so are able to keep up with their mother (horses, cattle,dolphins)or almost fully & completely developed (wolves,cats,mice,seals,bats). The young of viviparous animals are never able to fend to themselves immediately after birth. mothers can become awkwardly round before giving birth, which can slow them down when running from predators.
The viviparous animals that build nests can move their young to a new spot if the mother feels it is not safe. Because young are raised by their mother, they are better able to fend for themselves because they know the location of food and water, know the local predators, know the area, and have life experience.
 
No matter whether oviparous or viviparous, some species must build nests.