I don't think its necessary, at least not for the reptile. I feel that if the person has the space and resources to keep the animal in the same (appropriately-sized) enclosure for its entire life, then go for it. But I understand that if a person is aware of what is eventually required, but temporarilt limited on space or resources, then there's nothing wrong with keeping a young animal in a smaller enclosure...just as long as when the time does come for an upgrade, the keeper can deliver.
Most fishkeepers do this because they have a new fish they want to introduce to an already occupied tank, but they're simply waiting for the fish to achieve a compatible size where it won't be harassed. Since mixing different herps together are another ballgame from fish and is often discouraged (there are already threads about this issue), this reason is usually null for herps.
Many would argue that a 90 gallon tank for a single juvenile ball python or a room-sized enclosure for a 24" nile monitor may be excessive while the animals are so young, but I say if the enclosure is adequately decorated and furnished, and the animals do fine, then I see no problems.