The core task of many, many games is to move forward through a hazard-filled “tunnel”. (Examples: Half-Life, Sly Cooper 1.) This is economical from a storytelling perspective: you know fairly accurately what players will have seen at any given point, and you can closely control how they encounter each situation.
From a content-production standpoint, though, the tunnel design is fairly wasteful. The reward for making progress is to see what comes next, so you can't afford to have the environment look too repetitive. In addition, players move through each part of the world once and then go on, never to return. If you've played the vehicle levels in Half-Life 2, or the rail-grind sequences in the original Ratchet and Clank, you know that in addition to seeing each part of the level once, players tend to see it only briefly. This is expensive.
As computers get more powerful they are capable of displaying higher-resolution textures, higher-detail models, and so forth. Environments take increasingly more time and money to construct. As a result, game designers are trying to induce the player to spend more time in the same place. I think of this as “arena” game design.
Later Ratchet and Clank games dropped the rail-grind sequences because they weren't cost-effective. (Rail-grind sequences are back in the latest installment. Insomniac must have found ways to bring their costs in line with the rest of the game. You can probably guess some ways from looking at it.) Later Sly Cooper games adopted a hub model, with a large central world that had lots of missions set in the same environment. Games like Grand Theft Auto take this to its logical conclusion. There are no distinct levels, just a single, large world.
Designers also use collectibles to encourage players to spend time looking at all the expensive scenery. Urban Chaos, a game by defunct developer Muckyfoot that came out the year before Grand Theft Auto 3 had similar gameplay to the GTA series. It had powerups tucked away in corners of the worlds which would give your character permanent boosts in speed, strength, and so forth. Not unlike Crackdown, really, to name a more recent example. It would be amusing to make a collectible out of the world's triangles themselves.