Modern senses cluster around 'mediocre/ordinary' — 平庸 (mediocre), 庸俗 (vulgar), 庸医 (quack doctor), 庸人 (a nobody). The archaic 'use/need' sense survives in fixed expressions: 毋庸置疑 ('no need to doubt') and 无庸讳言 ('no need to hide it').
history
中庸, the Confucian classic 'Doctrine of the Mean' (one of the Four Books, 四书), uses 庸 in its older sense of 'constant, unchanging' — the title means 'centred and constant', not 'mediocre middle'.
