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its very simple man ......
resistor is to draw the current 4m the voltage source when 3 is detecting any device thus grounding the arrow..
it is because if u connect any voltage souce directly to ground then there will be short ckt !!!!!!!!
Actually, I didn't understand all thoughts that have been expressed previously, but no problem.
A similar resistor for sure isn't generally used with DC input connctors. I have designed a lot of devices, that never needed it. So it must have a particular purpose. Not knowing the remaining circuit or the application, I can only guess.
It could be, that the circuit has a design flaw and get's damaged when connected to a switched-off power supply but the device has it's internal decoupling capacitors yet charged. The resistor could prevent this by discharging internal capacitors. Also other device properties may require a complete discharge of input voltage.