The pilot needs to keep the aircraft balanced. This is really important. If you put all the heavy people at the front, it will push the nose down and the pilot will have to trim the controls against that, generating drag. If there is a sudden engine failure during take off, that extra drag could be extremely dangerous. The aircraft will also need more fuel to be loaded since the engine will have to run at a higher power to move the extra weight around. The aircraft will need to reach a higher speed on the runway before taking off. The aircraft will take longer to stop on the runway because it's heavier. The pilot needs to know all of this information precisely, in case there is a problem and he wishes to abort the take off.
On a large aircraft there are a large number of passengers and on average their weight can be estimated without weighing them individually. The standard adult weight is presently 87 kilograms. Some people will weigh more and some people less, but if you have 150 passengers, the average weight is good enough. So on a large aircraft it is not so important.
However it is not unusual to be moved around the cabin before take off to keep the weight distribution ideal. It was quite common on Concorde that a few people would be selected to move back a few rows, as that aircraft was enormously sensitive to balance.
Recently a Qantas aircraft got into difficulty during take-off because ninety children on board will accidentally assigned the standard adult weight instead of the standard child weight. This meant that when the pilots calculated the weight-and-balance of the aircraft before take-off, they got the answer wrong and therefore they used the wrong settings for the take-off. The pilot found the nose of the plane very heavy and he had considerable difficulty in performing the take-off without smacking the tail into the runway. It also meant that the calculated safety speed (the minimum speed in case there was a sudden engine failure) was also incorrect. Fortunately there were no further problems and the flight crew re-trimmed the aircraft during the climb out to relieve the problem. It was not a serious problem on a large aircraft. On a small aircraft it could have been very dangerous.
An aircraft is basically like a see-saw, balanced over the wings. Even a tiny change in the weight balance of the aircraft will cause it to tip forwards or backwards, pushing the nose up or down. If that isn't compensated for, the aircraft will begin to climb or dive. Therefore that change in balance needs to be accommodated by re-trimming the flight controls against the balance change.
Normally this will be done by the autopilot but if the autopilot is not available, it means the pilot will spend the entire flight constantly re-trimming the flight controls every time anyone goes to the bathroom.