Answer:
I only can answer the first part.
One situation where the slope can be important is for any proportional relationship, for example, if you know that with $5 you can buy 10 units of an object, then you can find the slope of the relationship and know the number of units that you get for any amount of money.
Other case is that, if the equation for your daily payment is:
y = a*x + b
where x is the number of hours that you work, the slope "a" has an impact on how your daily payment increases per hour worked.
