The OP gives a reason, and it's the same reason programs use types. Untyped proofs are usually wrong, like untyped programs.
[1] A. Church, A Formulation of the Simple Theory of Types. https://pdfs.semanticscholar.org/28bf/123690205ae5bbd9f8c84b...
[1] A. Church, A Formulation of the Simple Theory of Types. https://pdfs.semanticscholar.org/28bf/123690205ae5bbd9f8c84b...