What is the meaning of "for going on * years"?

I read a sentence such as " we've collecting materials for going on seventy years now " But, I couldn't get the meaning because I thought that it would go well without the "going on", and just "seventy years now" What does the additional "going on" make a difference... show more I read a sentence such as " we've collecting materials for going on seventy years now "

But, I couldn't get the meaning because I thought that it would go well without the "going on", and just "seventy years now"

What does the additional "going on" make a difference to the meaning of the sentence?
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