It’s always important to look for situations where a resource can be used or accrued in a nonstandard way or at a nonstandard cost. Such situations can let you get a better-than-market value deal by bypassing the usual exchange rate on that resource, so to speak.
In Civilization V, culture is a resource that can be spent to unlock social policies, which are powerful empire-wide bonuses. Normally, each policy you unlock costs more than the last, causing lategame policies to cost upwards of 10,000 culture. There are a rare few means of getting around the normal costs, with effects that unlock a “free social policy.” Free social policies are very powerful because they do not increase the cost of future policies. Stacking two or three free social policies by the midgame can make the cost of one’s 14th or 15th policy come tens of thousands of culture cheaper than normal.