3 ideas
Betting and raising give you two ways to win a pot: you hold the best hand at showdown, or your opponent folds first. Calling gives you only the first. So a player who keeps applying pressure, and makes opponents decide for all their chips, collects many pots they could not win on cards alone.
The player who acts after the others sees what they did before committing any money. That information lets them bluff more cheaply, take free cards, and fold without losing much. A good player therefore plays more hands in late position and far fewer in early position.
In no-limit, a hand's value depends on who you are playing against. Read each opponent's tendencies, patterns and physical tells to judge whether they are strong or weak. That read tells you when a marginal hand can win and when a strong hand is beaten.