Banks when they calculate the interest you owe them.. Or if you're lucky, how much they owe you.. So programmers may translate such an equation into a program so you don't have to worry about this kind of thing. So really you're kind of right... And this is it in a nutshell really, unless there's a specific need or interest then there is no need for the knowledge as there are tools that do it for us.
For example, if I were calculating interesting then the equation would look like this:
Let's say you borrowed £5000 at a rate of 5% for 200 days
Amount x rate / basis x number of days
Translates to
(Amount x (rate / basis)) x number of days
So you would do the divide first and so forth
(5000 x (.0500/365)) x 200 = 136.99
It would be a completely different answer if done from left to right..
(hopes i'm right )