Supply house statements never match the invoices we've actually entered
A tech grabs parts on the shop's account at the supply house counter, and the invoice either never makes it back to the office or gets tossed on a desk and forgotten. When the monthly statement arrives, half the charges on it have no matching bill in QuickBooks, and nobody can say for sure what was actually owed.
Someone ends up either paying the statement total as a lump sum without checking it, or holding the whole payment while they try to track down invoices from three different job sites, and either way the supply house account starts looking unreliable on your end.
A supplier that stops trusting your payment process will pull net-30 terms and move you to pay-on-pickup, which means a tech standing at the counter with a card instead of the shop's account, and that alone can slow down a job that was supposed to close out same day.