How is a message digest decrypted?
First of all signing does not equal encrypting. It only works on some crypto systems and even then it is not the whole picture. 

Hash algorithms are used for various reasons. One of them is to reduce the size of the signature since the digest is generally a lot smaller than the message itself.

But the main cryptographic reason behind hash functions is to make it so that a possible attacker cannot force the message he wants to forge a signature for, to have a certain property.

It is a bit technical but for example in the ElGamal cryptosystem if you sign the message itself then it is possible for an attacker to produce a valid signature without knowing the secret key. If however you sign the hash of the message  then this is not possible.