Reading 11 electronics

1. When two 16-bit numbers are added together in combinational logic (using a bunch of AND, OR, XOR gates), why do some bits of the sum require more time to  update (time w.r.t. the time at which the input bits are updated) than others?
1A: The reason it takes a while to calculate the sum of two 16 bit numbers is the propagation delay between each set of gate outputs. Each input must also be remembered by the SR latch, so there are a number of delays that occur to get the final result. 
2. What role does the clock play in a synchronous digital logic circuit?