Before data (a.k.a. "as received data") documents the accuracy of your data logger or chart recorder before it is calibrated. When your instrument is re-calibrated it is returned to original specifications. You cannot retroactively obtain "before" data.
Before data is required for ISO 17025/A2LA Accredited NIST Traceable calibrations. (N450 Compliance-Max).
Before data is required to accurately determine application-specific calibration schedules. (N550 Before & After or N450 Compliance Max)
Many regulations (e.g. QS 9000, FDA 21 CFR Part 11, CDC Vaccine recommendations, among others) require notifications of out-of-tolerance conditions. These in turn require before data, as do many companies? quality control manuals and standard operating procedures such as those specifying standards for various automotive parts.
If your application is subject to liability claims, the only way you can demonstrate due diligence in maintaining instruments' accuracy is by including documentation of "before" calibration data and step-by-step determination of required calibration schedules.