- In security terms:
The public key in all of the cryptography schemes is related to the private key. The only secure measurement that prevents a user to calculate your private key from your public key is the assumption that the discrete logarithm is "impossible to solve"
We all know that quantum cryptography is aproaching more every day, and the problem that this introduces is that solving discrete logarithms is "easy" if you are running a quantum PC/CPU. So having the Pk, it'll be possible to get the Sk(Private key).
In order to anticipate to that, if you only have acces to the hash of the Pk, you don't know it, so even with quantum crypto (as far as now seems that quantum won't be able to pass hash functions that easily), you are secure.
More info about Hash functions resistance to quantum crypto here.
- In gas spending terms.
The length of a Public Key is so much bigger than it's hash which is always a 256 bit variable. So if you count all the transactions that are done on a day, you are saving a lot of gas on transactions because the public keys will make the miners and the nodes process/store much more data because they are so much bigger than it's hashes.
I can expand more the explanation if you need it, but essencially, this is it.
AS you're asking for on the comments, here you have a topic on which is explained how to retrieve the Pk from it's hash.
Get public key of any ethereum account
Hope it helps.