Knowing your FTP, or at least a decent approximation, can be helpful for some things. In particular, it’s helpful for calculating TSS .
However, it’s totally unnecessary for being in the right training zones when doing intervals, and may actually be very counter productive. Lately, I’ve been simply tracking my interval workouts, and to progressively overload, I increase either the duration or the wattage.
So, if I’m doing a 4 x 6-minute interval workout, and doing the best I can (i.e. the most watts to successfully complete all 4 intervals), then I know I’m in the VO2max zone since the duration is in the 3 to 8 minute window. When I repeat that workout, I should be able to increase something - either duration (e.g. 6.5 minutes) or watts.
An advantage of this approach is it takes into consideration your unique power profile i.e. whether you’re a sprinter, time trialist, all arounder, or whatever. Maybe your anaerobic intervals (30 seconds to 3 minutes) need to be higher or lower than your FTP would indicate. By tracking your intervals, it just doesn’t matter what your FTP is, you simply want to progressively overload in some way.
I do occasionally test so that my TSS is reasonably accurate, but in reality, I could get a good approximation based on the IF of the interval workouts I successfully complete.