A VPN costs money to run. Having servers in multiple countries running constantly is costly. So consider this, if they aren’t charging you to use their servers, how are they paying for it?
When connecting to some VPN service providers, they will actually use your computer to serve others bandwidth. This will slow down your machine because your bandwidth and processors are actually being used by someone else. Additionally, if someone is using your computer for nefarious activities you could be held legally liable for the content of their browsing.
Some companies will collect data logs on your browsing, app usage, things you buy online etc and use it to their own benefit or sell it to companies who use it for advertising purposes.
Some providers will modify the VPN code to show ads. Again they monitor your use so the ads are targeted specifically to your browsing. However, sometimes the code can be very close to malware which can be very difficult to remove from your computer without doing a full wipe to clean it.
Disreputable companies can pose as a free VPN, this would allow them to monitor all of your activity and quite possibly gain access to your user accounts, bank information etc