Isometrics are useful for some things, athletically and for muscle growth. I have used isometrics, specifically yielding isometrics (like you described, holding a position rather than pressing against an immovable object), a lot in my lower body training with success. I feel like they have served a great purpose and as a bit of a side effect, I have very well developed legs.
Some exercises lend themselves better to this than others. I have probably had the most success with weighted Bulgarian split squats, picking a heavy weight that I could only hold for 20-30 seconds or so. I attribute them to helping put a lot of size on my legs.
Interesting study on yielding isos:
During submaximal eccentric, isometric, and concentric voluntary contractions, the AL increased linearly with increasing tension level. The significantly weaker slope of the AL-torque relationship obtained during eccentric contractions indicates that the voluntary activation reduction is not limited to maximal eccentric efforts but may be an inherent characteristic of the command to activate lengthening muscles voluntarily.
It, therefore, supports the fact that, during eccentric contractions, less neural drive (number or discharge rate of the motor unit) is needed to develop a given submaximal muscle tension (8). Identical findings have recently been obtained on human soleus muscle using a different technique, i.e., comparison between submaximal electrical activation and maximal and submaximal voluntary efforts (30).
Nevertheless, according to Pinniger et al. (30), this activation reduction would happen at least for torque levels >30% of the MVC and not for the whole eccentric torque range. The reduction of the motoneuronal excitability during eccentric contractions (31), possibly originating from presynaptic inhibition changes, may explain this result.
Indeed, it has already been reported that, during submaximal lengthening contractions, high-threshold motor units may be preferentially activated (27). However, contrary to the eccentric condition, submaximal concentric efforts indicate a similar relationship to that fitted during isometric contractions. Such similarity has already been observed using short trains of stimuli (28).
Thus a discontinuity between submaximal and maximal concentric contractions is revealed that is probably due to different neural mechanisms intervening in the activation regulation for the different tension levels. "
The study can be found at the following link