Earlier this week, PayPal agreed to purchase Honey, a Los Angeles-based coupon finder, for an eye-popping $4 billion. If it goes through, it will be the largest tech deal in the city’s history, and PayPal’s biggest acquisition ever. Why would any company shell out that much for a shopping tool? PayPal revolutionized online shopping with its payments system two decades ago, but lately more tech companies have been encroaching on its turf.