Many VPN companies these days offer both free and paid VPN options, but when you opt for the free one, you make some sacrifices. Most of the time, the free versions allow for fewer devices to be connected at the same time, slower speeds, fewer servers, and limits on data usage per month.
Some services, like ProtonVPN, offer unlimited data but will offer better speeds to paying customers during peak times. Windscribe allows you to score extra data each month by referring friends and tweeting about the service, so there are definitely ways to live with a free VPN, you just need to be a bit more aware of the limitations and factor those into your decision-making.
Upgrading to a paid plan ensures that you see no ads, which oftentimes means that there are no data limits, and many companies offer unlimited connections as well.
Any VPN (free or paid) is better than no VPN at the end of the day. Give a free service a shot to see if it works out for you, and if you are happy with it, then maybe you should consider upgrading to a paid option for an even better experience.