Why use FREERTOS instead of the interrupt mechanism on a microcontroller?
You said, "the goal of multitasking is to give the impression some sort of parallellization is going on on a single core".  That is way off the mark. I would say that the goal of multitasking is to divide a set of complex requirements into more manageable chunks while increasing the overall responsiveness of the system.

You might be able to achieve the first part of this goal using interrupts alone.  But if you put an entire task's worth of functionality into an interrupt handler then you're going to worsen the system's responsiveness.

Imagine a monitoring device that:


continuously processes samples from an ADC connected to a sensor and
performs some signal processing on the samples,
has a simple user interface with a display and a few buttons,
and has an Ethernet interface to communicate data on a network.


To do all of this from a main super-loop would make for a pretty complex super-loop.  You could simplify the super-loop somewhat by moving functionality into interrupt handlers.  But imagine if the ADC interrupt performs all the signal processing, the button interrupt performs all the display updates, and the Ethernet interrupt performs all the network message parsing.  This system would not be very responsive because it's spending relatively long periods of time in interrupt handlers.

With an RTOS, you divide requirements into tasks making each task relatively simple to implement.  Interrupt processing is kept to a minimum and the scheduler determines which task to execute.  This can make for a highly responsive system.  If you've designed the tasks smartly then it's also more robust and extensible than the complex super-loop.