Why SHA-224 and SHA-256 use different initial values?
This We called Domain Separation, when we use same algorithm for different output size.
Separation is necessary because if i found two messages which have hash value (SH256), differs only in last octet and then i can publish the hash value as first 7 octet showing i used SHA224. since i already have two messages colliding on SHA224 which i can use later for forgery attack.
Using domain separation we can avoid such situation