Trample answers the chump block. Without it, any creature on the battlefield, no matter how small, can stop an arbitrarily large attacker for a turn by stepping in front of it. With trample, the blocker only delays the overflow: assign lethal to each blocker, and the rest carries through to the player. The defender chooses whether to spend a creature to soak some of the damage or take it all, and neither answer is good once the attacker is large enough.
Because the keyword scales with power, it is close to free text on a one or two power body and decisive on a finisher. It belongs to the family of evasion abilities, but unlike flying or menace it does not let a creature attack through an empty sky; it punishes the cheapest, oldest form of defense, the one-mana token or the 0/1 mana dork. It also opens lines that pure evasion does not, particularly with pump effects and combat tricks, where any unblocked point of power becomes a real point of damage. Deathtouch piggybacks on it cleanly: one damage is lethal to each blocker, and the rest tramples over.
Trample on a small body is mostly decorative. The keyword does nothing about removal, nothing in a stalled board where blockers can profitably trade, and nothing on defense. It is a finisher's ability stapled, often, to creatures that are not finishers.