[j7ake]

Because advancing science and mathematics is not a straight line but full frustrations inside a cloud of uncertainty and anxiety. It is important to appreciate what the world looked like before the discovery and how someone tackled a problem to change this view of the world because when you yourself make discoveries you will be in the exact same situation.

However if all you care is to use what has been discovered, which is becoming less and less valuable, then you don't need to learn history of mathematics and science.

Although this is not good evidence but rather an anecdote, I cannot remember any significant person who has made fundamental contributions to mathematics or science that was completely ignorant of the history of the field.