There are several uses of Virtual Private Networks or VPNs but there are more.
The first use is the most common, this is where remote workers connect to their office or company network. However it is also used by some to access their home network and in the age of cloud computing is sometimes used to connect to cloud hosted projects. The second use is where people want to access content that is locked to a specific region and hence they connect to a VPN in that country to then "appear" to be in that country.
- Point-to-Point Tunnelling Protocol (PPTP) is obsolete and insecure, as the insecure nature is in the protocol and is proven this should not be used
- Layer 2 Tunnelling Protocol (L2TP) does not provide any encryption or confidentiality by itself and therefore IPsec is often used to provide confidentiality, authentication and integrity to L2TP. This is known as L2TP/IPsec where IPsec provides the secure channel and L2TP provides the tunnel.
IPSec is generally considered secure with a good algorithm like AES, it is believed IKE is exploited. If the IPsec pre-shared key is public then it is vulnerable to Man-In-The-Middle (MITM) attack.
There is also WireGuard: fast, modern, secure VPN tunnel which is a good alternative to OpenVPN.
If you use dynamic IP addresses and need to connect a VPN to them, then Dynamic DNS, like No-IP | Smarter DNS Starts Here can help, if the device you are connecting to supports it.