While 相当 (xiāngdāng) is a functional word, its usage reflects the Chinese cultural value of moderation and avoiding hyperbole, a concept related to 中庸 (zhōngyōng), the “Doctrine of the Mean.” In many situations, using an extreme word like 非常 (fēicháng, extremely) or 太 (tài, too) can sound exaggerated or even insincere. 相当 allows a speaker to give strong, positive feedback that sounds measured, objective, and thoughtful. In Western cultures, particularly American English, there's often a tendency towards enthusiastic exaggeration (“That was amazing!”, “It's literally the best!”). Using 相当 is the Chinese equivalent of offering a more reserved but equally strong compliment. Saying a meal was “相当好吃 (xiāngdāng hǎochī)“—”quite delicious”—can be a higher compliment from a reserved person than “非常好吃 (fēicháng hǎochī)” from an effusive one. It implies a considered judgment rather than a fleeting emotional reaction.
This is its most common usage. It modifies adjectives and some verbs to indicate a high, noteworthy degree. It is generally more formal and objective than 很 (hěn).
In this form, it's often used in the structure A 相当于 B (A xiāngdāng yú B), meaning “A is equivalent to B” or “A amounts to B.”