You could lookup the technical specifications for you specific drive.

External drives that are not host-powered (which have to be wired to a power outlet with an adapter of their own) have at least 4 different power measurements.

1. Active -- when disk activity is going on
   • Idle -- when disk activity is not going on, but the drive is attached and powered
     The drive may or may-not be spinning at this time (that could be two different sub-states).
   • Standby -- when drive is Idle, Attached but has gone into a standby power & wear saving state.
     This is when it takes a little longer to access it again.
   • Host-shutdown -- this is when the host-PC is shutdown but the drive continues to be powered through its adapter.
     This is similar to any other appliance in its 'standby' mode where it is shutdown but the power outlet is still ON.

The tech-specs for these drives will usually show reducing power requirements in ascending order for the above points. And, if the drive does not perform on one of the latter points, it would typically be excluded from the tech-specs (think of that as the fine-print).

As a bottom line, the states of your drive where the disk-platter is not spinning (assuming you are not talking about solid state drives here) will have relatively lower power consumption compared to when it is spinning.

This difference can be usually perceived by keeping your palm on the top of your drive (if its close by). A spin will always show up as a mild vibration.

The Maxtor (Seagate) drive you refer should have a good idle/standby control.
And the power consumption would typically fall significantly once it spins-down.
^{I hazard it be be around 2w tops after spin-down, but don't go by that :-)}
So, it should be fine to leave it wired and powered while your PC is on.
Unless, you keep your PC on most of the time and use the drive very rarely (or do not turn off the drive power when you shutdown your PC) -- in that case, extra efforts towards powering-down the drive when not in use would be worthwhile.

If your question is "Why do people buy large power supplies?" then there are a few reasons.

Their power requirements could actually be that high

Your computer has fairly modest power requirements. Power requirements are mostly driven by the CPU and GPU. Your i5-2500K CPU has a maximum TDP (thermal design power) of 95 W, and your nVidia GTX660 has a TDP of 140 W. Given your particular hardware it's not surprising that you observe 200 W as the maximum load. A 300 W or 450 W power supply would be quite adequate.

However, other people's computers can have much higher power requirements. A computer with a 150 W CPU and 2×250W graphics cards will need at least 650 W, not including the disk drives, USB devices, fans, water cooling pumps, LED lights (...) also drawing power from the power supply.

Such a computer would generally be equipped with a power supply in the 800-1000 W range.

They are purposely buying a bigger power supply than they need

Even if the computer's power requirements are only (ex.) 300 W, some people would go out and buy a 600 W power supply for it anyway.

The reasons they perceive this might be a good idea include:

1. To allow some headroom for future upgrading of the computer - new parts might need more watts.
2. So that the power supply will run at a lower temperature. Components degrade faster at higher temperatures, so the power supply is less likely to fail.
3. To run the power supply at a higher efficiency. A power supply run at 100% of rated load is actually less efficient than a power supply run at 80% of rated load.