I don't think it is necessary, but a working understanding of it and how microcontrollers function can be very helpful for debugging and adapting existing code.
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What @Community member said. And not just for STM32 - for any microcontroller.
Note that a specific design goal of the whole Cortex-M family was to enable you to do your whole project in 'C' - without needing assembler glue to hold it all together.
It is not essential, but at some stage a few small bits of assembly language in an inner loop can double/triple/.... your processing power (or/and increase your battery life). - and the assembly language of these ARM chips is so rich and powerful - especially in the M4 series. I enjoy writing the occasional bit of assembly - but only in small doses
