Purity tests came into existence as a way of enabling people to accurately judge a person's percentage of sexual purity. A purity test by its very nature is a tool by used to test purity of the person taking the test. Among societies that rigidly adhered to strict guidelines as to what constituted proper sexual actions and what didn't such tests were used as a means of control.
Many ancient societies did, and some still do, hold to the concept that sexual activities should only be indulged in when work was being done. That 'work' of course was the forming of a new human being with the ultimate societal result being the continuation of the species. In those groups subscribing to a patriarchal line of succession the women were expected to remain virginal until the night of their wedding at which point their husbands could be assured of forming a child that would carry on their genes.
Interestingly enough while the women were supposed to test purity positive in order to be considered worthy of attention and not punished as being the lowest scum of the earth, men rarely if ever had their percentage of sexual purity tested. It was considered the norm for a male so 'sew his wild oats' and even to this day while it's not outwardly condoned there is still a greater stigma attached to a woman who cheats than a man.
Historically this type of judgment comes about due to an attempt on the part of those in power to keep control over those they wish to keep in abeyance to their desires. The other side of the coin comes again from the idea that procreation and species survival are the most important result of sexual interactions. However there are many naturally occurring sexual unions that don't result in birth. An example of this is homosexuality. While completely harmless certain rigid societies didn't want their members wasting their energies on frivolity-as they perceived it to be-but wanted everybody to have sex with one end result in mind. Having children and keeping the group going.
Another reason for checking on percentage of sexual purity and keeping ones subjects in line revolves around wanting to have a large labor force. More bodies, more work, a better community and so on and so forth. There is also a lessened chance of spreading disease if casual sex partnerships are discouraged. So in societies that deemed it necessary to exercise rigid control over the sexual activities of at least one gender of the community ways and means of testing percentage of sexual purity was a crucial tool in measuring those worthy against those that weren't.
A purity test is said to be a set of questions that when answered honestly, approximates the degree to which the person taking the test has been corrupted by a lifestyle or activity. As already mentioned, the first tests were designed to test the degree to which the test taker had been "corrupted" by sex. And of course the desired score of percentage of sexual purity would be 100%. It's interesting to note that the same amount of attention as has been paid to having sex hasn't been given to the other naturally occurring and no less vital bodily functions we 'indulge' in every day.
Breathing, chewing, digesting, sweating are all normal bodily activities but none of them has had so much attention and so many traditions and tests created as a result of their machinations as has the activity that produces a child. Although still used in certain countries, the purity tests of today are usually less about implementing control and more about providing information and entertainment.