Ground Wire to Prevent Static Discharge

Because electric current is due to the flow of electrons which carries negative charge

You can ground either positive or negative end of the power supply and electrons will temporarily flow until the electrical potential at that end becomes the same as the electrical potential of the ground; the potential of the other end of the power supply will maintain the same difference relative to the grounded end as it originally maintained relative to the non-grounded end. The non-grounded situation is sometimes called a "floating" power supply, in contrast to the "grounded" power supply.

If you ground both ends of the power supply, you short out the power supply; electrons will flow to try and create equal potential at both ends of the supply until the protective elements (circuit breaker or fuse) trip to prevent overloading and burning out the power supply.

Updated response to your comment. With one end of the power supply grounded (either end), electrons flow between the grounded end and the ground to equalize the electrical potential between that end and the ground. Inside the power supply, the electromotive force (emf) used to maintain the potential difference between the ends of the power supply (battery chemical reactions, generator action, whatever) results in charges being redistributed internally in the power supply as necessary to maintain the same potential difference between the two ends whether one end is grounded or not. Yes, the potential of the ungrounded end relative to ground can be different than the ground potential if the other end is not grounded, but the potential difference between the two ends is the same whether one end is grounded or not.

See the following example; the box indicates the power supply, the top is the + end voltage and the bottom is the - end voltage. Three conditions are shown: power supply ungrounded (with floating potentials $V+$ and $V-$), negative end grounded, and positive end grounded. The potential of the grounded end of the power supply changes from its floating potential to equal the ground potential $Vgnd$ but the potential difference between the two ends of the power supply, $Delta V$, does not change, no matter which end of the power supply is grounded.

Grounding the negative side of the DC-powered circuit is more a matter of a convention.

You can ground either and get the same effect.

The chasis will not be able to achieve a thousands or tens of thousands of volts static potential difference from your circuit, so no sparks possible.

The convention about grounding the negative side brings two benefits that may or may not be important:

1. One can clip the negative voltmeter conductor to the chasis and measure voltages in the circuit. This way, one will almost always get positive measurements. Most electronics are negative-ground, too.

2. In some cases, involving different metals, intermittent moisture and/or other factors, the electric potential affects corrosion. The chassis is somewhat safer against corrosion when connected to the negative (the positive voltage promotes the electrical conductor's corrosion, but the conductors are easier to replace than the chassis.

The effect is quite visible in marine and automotive environment. A lot of retro cars built pre-1965 have positive chassis. 50-60 years later the wires are "like new", but the steel around them is... well, worse than "like new".