Contracts are often not concluded by lawyers. It is therefore not uncommon for the contract that you thought you had concluded to be a different contract than you thought. The party you thought you were doing business with turned out to be different. “Suddenly” terms and conditions may apply that you never wanted to agree to. Or your own general terms and conditions do not appear to apply.
In addition, there appear to be matters about which no agreements have been made that subsequently prove to be of great importance. Or matters on which agreements have been made become so restrictive for you due to changing circumstances that the contract must be adjusted. It often turns out that you have more or less rights than you thought.