Their purpose is to implement Boolean Logic. You might notice the correspondence to Binary Semantics and thus Binary (number system) logic & mathematics. Special fun: OR and XOR (exclusive OR).
Geographical Considerations: While most contractors will price their asphalt shingles roof replacement services within $4.00 to to $7.00 per square foot or $400 to $700 per square range, there will be significant geographic variations.
Most ranch type houses in the US, measure an average of 15 to 20 squares in terms of their actual roof surface, which translates to $6,725 to $9,000 for the very basic composition shingles roof installed, based on the average installed cost of $450 per square, with a typical 5 years labor warranty.