Why do parrots mimic human speech?

Answer: They copy sounds to strengthen bonds with flockmates and caretakers.

They must understand a sentence before they can copy its sounds. — Copying speech does not require understanding the sentence. Some trained parrots can learn labels for objects and use them appropriately, but that is a separate question from why they imitate voices. Socially learned sounds help parrots interact with the individuals around them.

They copy sounds to strengthen bonds with flockmates and caretakers. ✓ — Learning familiar voices can help a parrot maintain social contact. Wild parrots learn calls from other parrots, and captive birds can learn sounds from their human caretakers. This links speech imitation to social learning without implying that every repeated sentence is understood.

They mimic sounds to lure prey and improve their hunting success. — Parrots do not imitate human speech to lure prey. Their learned calls help them recognize and contact social partners, not improve hunting. With people providing much of a pet parrot's social contact, human sounds can become part of its repertoire.