It helps to keep things organized
Let's say for example that the business is taking in $2000 of revenue. That is the amount that the business collected for it's services - like for fixing the computer. What if though it costs $500 for the equipment (that's an expense). Now they only made $1500. Now the customer complains and says that the computer isn't fixed properly so the company sends out a techie for 2 additional hours. They need to pay their employee (another expense). Now the $1500 is down to $1400. They would have utilities to keep their lights on and insurance and many other expenses.
Your profit looks like this:
Profit = Revenue - Expenses
3-4 minutes is about right is say
