The metadata retention scheme requires the storage of:
- Connection open time and duration
- Your location
- Total amount of data sent/received during the connection
- Your IP address
- * Does not require collecting the destination IP address, but it will be more effort to strip this out with lots of tools
So if you're web browsing lots from home or making lots of connections to servers an ISP has to store lots of records. However, they only need one database row for your VPN connection!
Then ISPs could offer discounts to VPN users as they reduce the amount of metadata needing to be collected.