GPG seems to be something that would be important to people who communicate to a particular person and have to keep their privacy and/or security. But what if I'm just a casual user who's aware that my privacy is getting invaded, is there a use for GPG for someone like that, someone who's not being targeted for something nor is communicating with people who are willing to use GPG? Or is it just useless in that case?
nor is communicating with people who are willing to use GPG
PGP/GPG or S/MIME provide end to end encryption and tamper resistance. But to offer this it needs to be used by both ends of the communication. If the other end is unwilling to use PGP or S/MIME then no end to end encryption is possible. This is the same with HTTPS: if the web server does not offer HTTPS then it does not matter if your browser would like to encrypt the connection, it will simply not work.
GPG might still be useful as a general encryption tool for yourself, but it will not be useful to protect your communication if the other end is unwilling to communicate with you in an encrypted way.
First of all, PGP/GPG can be used for two things:
When encrypting a message, the end point (the user you're sending the encrypted message to) is required to have PGP/GPG as well in order to decrypt it.
Signing is used in order to know whether a message has been tampered with in transit.
In this case I'll assume you're talking about encrypting your email messages.
In my opinion, PGP/GPG can (and should) be used by anyone that is willing to use it. As mentioned before, the only down side is that the person(s) you're sending encrypted messages to are required to use PGP/GPG as well.
I use gpg/pgp for various common and trivial things including encrypting files, signing emails and encrypting emails, as well as some more esoteric things like time stamping documents for legal purposes.
My take on it is why wouldn't you? It means you are prepared for those occasions when you have to use it, your behaviour pattern already includes it, and its kinda cool (in geeky circles:-)