re:

In a test match (5 days) or these 2 or 3 day games the winning team must score more runs than their opponent AND get all the opponent's players out twice. If neither team can satisfy these requirements in the alloted time, the match is drawn.

Because there is a time limit, a batting team may decide to forfeit their remaining wickets and declare their inning done (after presumably amassing a high score). This affords them time to dismiss their opponents in time to win the match. There are opposing pressures at work: leaving sufficient time to get the other team out and scoring enough runs so they don't surpass your total and win the game. The responsibility or when to declare falls to the batting team's captain.

Hope that helps.