Tier 1 begins a campaign with no vehicles and no power weapons. At best, you tend to see a couple of precision weapons (Needle Rifle, DMR), but that is about it. Tier 1 is essentially a grunt's job of walking the long mile, a duck shoot, an in-your-face CQC melee in the trenches of the Objectives' Zones. It is during Tier 1 that the forger need not worry much about vertical travel, such as Falcon, Banshee, or Jet Packs - walls and barriers do not need to stretch very high to be effective.
Tier 2 introduces vehicles (usually light vehicles), and power weapons. Loadouts expand with capabilities, both in weapons and armor abilities.
Tier 3 introduces heavy vehicles and aerial vehicles, along with more power weapons. Loadouts are maxed, and Jet Packs are available.
In each Tier transition, we witness escalation along every category. We witness more vehicular power, weapon power, armor abilities, load out weapons, and play space (first the length of the map grows, then vertical play grows).
This is not by accident. If the game did not demonstrate an escalation of Game Play Experience, then players could find Game Play stale with the second and third Tiers playing like the first. And with each successful Tier win, players need to feel rewarded, and they need to experience an increase in the power of the Game Play. Invasion was designed to do all of this with each Tier win.
In fact, some players purposefully lose Tiers 1 and 2 so that they can get access to the Scorpion or Banshee. These intermediate rewards are more important to them than the score. They are more interested in the fun of the kill using more kill power and more explosions than winning the Game.
This reward system has two purposes. The first is to add cohesion between the Story and Game Play - that is to make the Story come alive and the Game Play seem logically progressing as a campaign. But the second reason is a far more important reason. It is to form a progressive pattern of Game Play with the ultimate end being the climatic win of the Core crossing the finish line.
Everything about the Game Play must continue to grow the players' experience, continue to move toward a climax, and continually increase the fun factor for the widest audience possible through the introduction of power weapons, vehicles, and loadout options.
Summary
The Game progresses to increase weapons, vehicles, loadouts, and play area with each Tier transition.
The escalation is to help bring the Story alive, to reward players, and prevent boredom.
The ultimate goal of this escalation is to reach a winning climax with the Core crossing the finish line.