Purely customization. Long-term players of 3.5 started doing wacky multi-class combos to make their character more unique. WOTC chose to keep it simple with 4th ed and i think successfully streamlined the rules in 5th. Pathfinder chose to add greater customization to each player class so that while multiclassing is still an option, a straight class PC can still be quite distinct.
Purely customization. Long-term players of 3.5 started doing wacky multi-class combos to make their character more unique. WOTC chose to keep it simple with 4th ed and i think successfully streamlined the rules in 5th. Pathfinder chose to add greater customization to each player class so that while multiclassing is still an option, a straight class PC can still be quite distinct.