It proves you were there
A property manager phones a four-truck mechanical shop and says your guy never showed up Tuesday, so we're not paying for Tuesday. The owner asks his tech. The tech says he was there. Now it's one man's word against another's, and the contractor almost always eats it — because arguing costs more than the invoice.
With tracking he opens one report: arrived 9:12, left 10:48. The argument stops before it starts. He gets paid, and he keeps the contract at renewal because he can show the whole year's visits.
"When a customer says nobody showed up — what do you say back?"