Not all decryption is doing exactly the same things in reverse. For example, with CTR mode (and thus GCM mode, which is CTR plus GMAC), you call the /encryption/ routine regardless of whether you're encrypting or decrypting data. This means in an embedded environment you can save die space because your program doesn't need the e.g. AES decryption routines too.

https://upload.wikimedia.org/wikipedia/commons/4/4d/CTR_encr...

https://upload.wikimedia.org/wikipedia/commons/3/3c/CTR_decr...

(Note the bold text)