It is the equivalent of checkdisk on a PC and checks and corrects or deletes corrupted file allocation table entries and the integrity of the recorded programmes. Doing this it clears up incompletely deleted files and can recover lost "free space". Certainly worth doing every few months or whenever the box starts reacting slowly.
It also causes the box to be cleanly reset - ie it stops any recordings including anytime ones - shuts the HDD down properly and resets the hardware and firmware.
A power-off at the mains power on cycle will also achieve a reset but carries the risk that the disc is being written to at the time of the power off - with attendant (admittedly very small) risk of disc corruption.
Periodic rebuilds don't do any harm and can help to reduce the risk of unexpected box crashes and/or failed recordings - it's basically like rebooting your PC on a regular basis - it helps to keep the memory and disc clean and functioning well.
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A power-off at the mains power on cycle will also achieve a reset but carries the risk that the disc is being written to at the time of the power off - with attendant (admittedly very small) risk of disc corruption.
Periodic rebuilds don't do any harm and can help to reduce the risk of unexpected box crashes and/or failed recordings - it's basically like rebooting your PC on a regular basis - it helps to keep the memory and disc clean and functioning well.
Not as many has Windows, that still needs rebooting periodically to keep peak performance unlike Linux