Mostly the simplicity of the account creation and the ease of automation (for creation and usage).

Additionally, Proton and Tuta explicitly label having multiple free accounts as an unauthorized usage of their services. https://proton.me/legal/terms (2.7) https://tuta.com/terms (4.7).

I don't think it's something they enforce. Nevertheless they could anytime, and meanwhile you might be happier by following the “Value4Value” principle.