Why do economists pace such emphasis on the concept of marginal cost
and should managers place the same emphasis on it in their decision
making

The marginal cost is the cost of producing 1 more unit.  
The marginal revenue is the revenue received on that same last unit.
If marginal revenue > marginal cost then that unit should be produced.

It's not always obvious what marginal cost or revenue will be so it's
not always a useful tool, but if perfect information is available
about costs/revenues then every manager should place a very large
emphasis on this concept because it can clearly tell the manager how
much of a good to produce.