This is done to prevent that Internet resources are accessed using the device's regular IP address.
The two remaining plans, ProtonVPN Plus and Visionary, for €8 and €24 offer the highest speed, five or ten devices that you may connect from simultaneously, as well as extra features such as Plus servers reserved to these plans, Secure Core which adds extra protection against VPN compromise by routing through the Secure Core Network of ProtonVPN, and Tor Server support to send all traffic through Tor with a single click.
Basic, Plus and Visionary subscribers get access to other servers, and there is likely a cap on how many can connect to these at the same time to provide good transfer speeds for all users. The most likely explanation for that is that there is no transfer speed cap, but that the free account is limited automatically based on the number of users that connect to a server. There is no information on what this actually mean in terms of transfer speed.
Low for free accounts, high for basic accounts, and highest for the Plus and Visionary subscription. ProtonVPN has three speed tiers right now.
It lets you connect to all servers, supports connections on two devices at the same time, and offers high speed. The first paid plan, ProtonVPN Basic, is available for 4€ per month.