As everyone has learned from Breaking Bad, higher purity means a higher street price. People don't want anything mixed in with their poison.
In Dr Meth, purity is a key feature. You can raise your purity by buying more expensive places, by getting the purity upgrades, and by getting a high combo from the super chemist.
While lower places bring down your purity average they do so in proportion to the amount of product they produce. For example while a hundred trailers might bring down your purity if you only own the trailers and one house, if you own a hundred houses and a hundred trailers they won't make much of a dent in the purity.
Efficiency plays a key role to purity. If you have too many of one place the purity of that location will go down