Because things change all the time, thus your risks are changing all the time. In order to keep the environment healthy and safe, you must review risks in a timely manner to keep up with the changes.
Like 6dogs said, because hopefully the risks are changing. Indirectly, it also demonstrates that whatever you are doing to counter risks is having a positive effect; or there is a new factor that is having a negative effect.
Yes they do - as and when.
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