Depressions are ideal for extremely large single-dish telescopes like Arecibo and FAST for several reasons, but the single greatest advantage is structural. These instruments are several hundred meters in diameter and therefore require delicate support systems to maintain their shape and avoid collapse. You could certainly try and build a freestanding dish, but it would be even harder than it already is. The karst depression FAST was built in, Dawodang, is close to spherical, like the telescope, making building the support structure comparatively easy (Zhu et al. 2018).
Besides the structural benefits unique for these large dishes, building a telescope in such a depression provides natural shielding from radio frequency interference. Some radio telescopes built on flat ground, like those at the Green Bank Observatory, are similarly protected by the surrounding hills (as well as, in Green Bank's case, a large radio quiet zone). Both Arecibo and FAST benefit from walls of their depressions -- one reason Arecibo's current site is considered promising for the telescope's possible successor, regardless of configuration (Anish Roshi et al. 2021).
Finally, depressions provide drainage. Water can flow to the center of the natural bowl and, particularly in karst topography, subsequently seep into the ground -- one advantage they have over other types of depressions, like craters or mine pits. While karst is permeable, it isn't fragile, so you don't have to sacrifice stability for the sake of drainage.
Basically, the structural advantages of depressions are applicable just to these extremely large telescopes, but there are side benefits, albeit ones which other, smaller radio telescopes not located in depressions can achieve in other ways.