Of course this service is free of cost to use - but it is not free of cost to operate and to offer it to you.
Supporting this service voluntarily by some small amount of money helps to provide this service as it is now.
Please remember that the #fediverse mostly consists of people that are committing their spare time and personal money to provide others with a free service.
On Twitter, on the other hand, you have adds and your data is being sold to other company. But not here.