Ok, also look at STM32L4 family.
First, at boot, the STM32 boots with internal resonator (HSI = high frequency source internal clock) say 16MHz.
It has some tolerances (percentage) so its precision has limitation in some specifics such as RS232 at high speed, USB, Ethernet.
For such precise clocks, you need 24MHz for USB, 25MHz for Ethernet or 16MHz typically otherwise.
The external clock source in Nucleo board by default comes from on board STLink MCU which sends its buffered clock to the target MCU (cost reduce).
These are HSE (precise external high frequency source).
If the HSE clock fails, the HSI will kick in as backup and the MCU can be let aware.
Now for specific applications you need precise time clock and calendar, this goes to RTC (real time clock).
There is an internal one (LSI) which allows some peripherals or the MCU to run at very low frequency aiming at low power.
But if you implement a meter application, you need very precise clock over time, and consume low power.
For this, you need very precise clock which comes from an external 32768Hz external clock source which is LSI.
On the L4 family, it is possible to regenerate a high frequency from LSE at lower power budget than using external dual crystals.
This is MSI.
So it's application dependent. Power concern, precision concern, cost concern, drift over time concern.
And like in the L4, most peripheral can choose their clock source. Now you have choice and enough degrees of freedom to get a solution for your implementation.
