When a rider needs the receipt
Business expense claims. The main reason. Employers reimburse against it.
Reconciling a card statement. A rider checking an unfamiliar charge needs the trip detail to identify it.
Disputing a fare. Surge pricing and toll charges are the two most commonly queried lines, and both need to be visible to be discussed.
The $75 rule. Under IRS Publication 463, US travel and transportation expenses under $75 do not require documentary evidence, and most single rides fall under that. You still need a written record of amount, date, place, and business purpose. Lodging is the exception that always needs a receipt, covered on the hotel receipts page.
A ride receipt filed as an expense receipt is small enough to be waved through and easy enough to lose, which is a good reason to keep the receipts as they arrive.