Negative thoughts are pointless?

I think I heard on a coaching call that negative thoughts cannot create positive results which made me think that we need to adopt purely positive thinking in order to achieve a specific result. Is that always so? I understand that life is 50/50 and appreciate that sometime we want to allow and experience ‘negative’ emotions such as grief and sadness but if they cannot create a positive result what is the point? Positive thinking is the key? Slightly confused.