What is the purpose of swept back wings?

Swept wings, however, are designed to reduce turbulence by slowing down the air as it moves across the surface of the wings. As previously mentioned, swept wings are longer than straight wings. Therefore, air moves more slowly across them, which reduces the amount of turbulence the airplane encounters.

How does sweep effect lift?

Forward sweep causes the tips to increase their angle of attack as they bend. This increases their lift causing further bending and hence yet more lift in a cycle which can cause a runaway structural failure.