It doesn't have to be, but it's more convenient. Adding extension lines to the supply hoses is one more thing to fail. You'll take up more space under the counter with the longer trap arm, and depending on how far away it is, you may not have enough height to achieve the proper slope. But, within reason, it's doable. Each layout will be slightly different, so you'd need to know where things actually are. Also, sink depths can have a big impact (related to the slope of the drain line) - if the new sink you want is deeper and further away from the wall connection, you may not have enough slope for it to drain properly.
