Why? In most cases, they will not help you.
Why? For example, you may want GND/PWR separate CLK signal from the others, it helps minimize crosstalk between signals on a cable. You may want to consider how currents will flow. You may want to prevent board damage when the mating connector is inserted the other way round
Why? Assembly house uses them.
Why? Helps you to identify symbols and footprints which you need to check, helps to speed up schematic drawing process
Why? It helps with power noise filtering, easy to measure currents, circuit can be disconnected, …
Why? In some cases you may want to use exactly the same components because of software / hardware compatibility. It may also help to prevent possible problems (there may be a reason why they chose particular chips).
Why? It melts at lower temperature and behaves better than un-leaded tin
Why? It helps you to find possible problems before mass production
Why? It helps you quickly jump to the right page
Why? Connector pins only can carry limited current.