If you intend to use the LSI as the source clock for the RTC you can apply a coarse calibration when calculating the prescaler for RTCCLK. Adjust the synchronous divider to vary the number of LSI ticks per second (fewer to speed up RTCCLK, more to slow it down). It's not ideal but you can get a reasonable time of day sing the prescaler technique.
As to why calibrate LSI? The IWDG watchdog uses it, so if you like to touch the watchdog just in time you need a fairly stable source to handle tolerances between parts. And if you rely on LSI as the fallback if LSE fails then you need a calibrated RTCCLK source. ST recognized this , which is why the STM32L4 series does have hardware LSI calibration.
Jack Peacock
