[derefr]

So, for a term of a contract to have force, is it necessary to have some sort of penalty in the contract for breach-of-contract, involving e.g. some required payment, so that you can then sue for failure to pay the penalty? (Which I would assume is mostly equivalent to e.g. failure to pay for goods transferred or services rendered.)

[TangoTrotFox]

No. Contracts typically do not have penalties. Civil cases are about lost value. The typical example is we agree for me to install copper piping in your house. Instead I install lead piping. Lead pipes cost $40, while copper pipes cost $60. I'd be liable for the $20 of lost value. Interestingly enough you generally could not even recoup the cost to remove and reinstall new piping. It's precisely about provable and direct lost value.