He explained that most protocols he has wanted to build on Bitcoin have been limited by the inability to concatenate stack values. Andrew Poelstra added to this discussion by outlining the potential uses for OP_CAT, such as creating Merkle trees and manipulating signatures. Additionally, OP_CAT would allow users to enforce proof-of-work-like computations on hashes or curvepoints and switch on hashes. While Poelstra did not have specific applications in mind, he believes that many lines of inquiry have been cut off due to the current limitations.