The original receipt from a luxury brand purchase is one of the most valuable documents a collector can have. It proves the bag was purchased new from an authorised retailer, records the original retail price (significant for Chanel and Hermès where pricing has increased dramatically), and provides a provenance trail that's difficult to fake.
Many collectors store receipts securely alongside their bags. When selling, a receipt can increase sale price by 10–20% and can be the deciding factor for serious buyers who require verified provenance. Digital receipts (now common with many brands' in-store systems) should be saved as PDFs alongside photographs.