Why do people tend to consider their firstborns as so important?

Question

I mean in the bible they killed only the firstborns and is like the firstborns are very important. Why?

Answer 1

proof of fertility , first borns specially the males  are expected to grow up to be leaders, inheritors of the family business, family name , bloodline. many cultures value sons over daughters. ( not me !!)

Answer 2

I think it's because, well in today's society, peoples first baby means ever so much, and that means a lot to them,(their 1st child),  doesn't necessary mean they don't / won't treasure their other children later in life...