What a billing machine does well
It turns on and it prints. There is no login, no update, no internet and nothing to configure beyond entering your items. For a counter selling a limited range for cash, it is genuinely hard to beat, and a shopkeeper who wants a printed receipt and nothing else should probably buy one rather than software.
What it leaves out
Everything that is not the receipt. There is no party ledger, so credit goes back into a register. There is no stock, so ordering is by eye. There is no purchase side, so what you owe suppliers lives on a spike of invoices. And there is usually no way to get the data out, so the machine holds a year of sales that nobody can analyse and that vanish when it fails.
The item list is the hidden cost
Entering and updating items on a machine keypad is slow, which means most shops enter a handful and then bill everything else as a generic line at a typed price. Once that happens the sales data is meaningless — you know the total and nothing about what sold. That is the point at which the machine has stopped being a record and become a calculator that prints.
Where software costs less than the machine
An Android phone you already own plus a Bluetooth thermal printer is a complete counter for less than most billing machines, and it brings the ledger, the stock, the purchases and the reports with it. If you later want a computer, it is the same account. The machine cannot grow; the phone setup can.