First of all this whole tutorial isn't a mandatory mod - you just need it if you run into issues (weird things are happening and you've already ruled out the more common suspects) or if the always running fan and illuminated display (even when the printer is turned off) disturbes you.
I'm starting with the why part:
If you connect a 3D printer via USB to your Raspberry Pi it can lead to all sorts of power related issues.
Most of the time those issues occur only if you turn on your pi before you turn your printer on or if you turn your printer off while the pi is running.
Which issues can occur when you got an USB related power issue?
Unfortunately it can lead to all sorts of problems - here some examples:
What we see most common are undervoltage warnings, unstable Wifi, non working or freezing cameras and disconnecting/malfunctioning USB devices.
We also had some indefinable problems like the print ending mid print which are difficult to diagnose because they could have different causes.
How can I prevent this issues from happen?
That's pretty easy 
You simply cut a small stripe of tape and put it on the 5V pin of your USB plug.
Here you see an image of the standard USB A and B plugs.
Pin 1 is the 5V pin which you're going to cover with the tape.
Here two pictures if you feel unsure what to do:
Cut a stripe of tape and grab it with a pair of tweezers.
and put the tape on Pin 1 of your plug
I like to let the tape overlap a bit so it doesn't move when the pins of the USB port connect to the pins of the plug.
What happens if you do it wrong?
Don't worry - the worst that can happen is that you can't connect to your printer anymore because you covered the wrong pin(s). Just remove the tape and put it on the right place and it will work again 
Does this work for any printer?
While it will work for the most printers there are some exceptions. Those printers need the 5V or the printer won't connect to your pi. The only printer I know for this exception is the BQ Hephestos 2 but there might be more.
