Supplier rules can make both your life, and your clients’ lives, easier by automating much of the data input process.
For some suppliers, things like category, tax, due date and payment method are unlikely to change.
It’s unlikely that purchases from McDonald's will be made on credit or that they will be of anything other than food, so a default category and payment method can be set.
Depending on the format of the receipt given to you when you buy from a certain supplier, some information like tax amount might be missing. If this is a recurring problem, it’s simple to set up a supplier rule to fill in the missing information.