Why do different companies have deadlines for bills to be paid up by it's customers?

Answer 1

Companies have bills to pay too, and need the income from their sales to customers to pay them. So they put payment deadlines so that people pay in a timely manner. Not all companies have the same deadlines, but 30 days to pay is pretty common, although many demand payment immediately.

Answer 2

If they didn't people would put off paying them and they would never get paid.

They have bills to pay, too and employees to pay. They have the right to expect to be paid for their goods/service in a timely matter.