The whole point of a VPN is to hide your traffic or obfuscate your traffic from trackers, ISPs, prying eyes, etc. That's why you pay companies to use their servers and their ISPs to route your traffic.
Good VPN companies will have thousands of VPN servers throughout the world so you can choose where your traffic comes from. Good VPN companies will also encrypt/hide your traffic so it is hard to identify you to what you are connecting to.
If you are using a VPN and logging in with an account at a website - that defeats one of the purposes of the VPN because the company that houses the account will know it is you.
VPN'ing to another PC on your local area network then having that machine send out its traffic would be pointless imo. There are some people who set up VPNs locally on their network just to encrypt traffic for a higher level of security (above and beyond https).
Hackers will often use a VPN to a VPN to a VPN to a VPN to try to mask their traceability.
Again, a VPN is usually used to secure traffic when security is non-existent - such as a public WIFI hotspot at your Starbucks, college, or Airport. While HTTPS traffic is encrypted, a lot of other traffic is not - and packet sniffers on the public wifi can easily steal that information. VPNs ensure that the traffic between your computer and the VPN server is encrypted.
VPNs also allow you to change the location of your Internet requests. For example: Netflix in Peru has far more show offerings than Netflix in the USA (I have relatives in Peru). So one can get a VPN to a server in Peru. Problem is, VPN companies aren't keen on paying for all that bandwidth usage - because that affects other people trying to use that VPN in Peru too - that's why on average, most VPNs have terrible speeds because some people use it to stream music/videos/etc - but someone pays for that bandwidth and server and it affects other people trying to use it. That's why there aren't free VPNs that are all that "good" in a sense.
I'd highly recommend taking some courses on internet security. Most HTTPS traffic is encrypted and as long as you have a normal ISP - VPNs usually aren't needed or even recommended. Using an up to date OS, with an up to date browser, with an adblocker usually does wonders. But again, even in that setup, if you're going to bad sites - no amount of antivirus, adblocker, up to date OS patches are going to stop infection. Thus it is important that whoever is operating the computer be educated first and foremost. This is why most security experts don't even run antivirus software or adblockers - they know what to do to avoid that stuff in the first place - most run throwaway virtual instances.