Buying a purebred dog
Humans often buy purebred dogs because they think they know what to expect from them – regarding looks as well as characteristics. That is not completely correct since, in time, with influence of dog show requirements as well as unnatural breeding alone, breeds change. This changes their appearance, introduces anomalies, not just physical but temper ones also. Therewith, from a dog that is "cleaned" from breeds you can expect more dog-like behaviour than with purebred.
Adopting a purebred dog
If you adopt a purebred dog from a rescue centre or you found it, rescued it... it is same as with adopting any mongrel dog. In this case, making distinctions would be doing "positive discrimination". But, requiring a purebred dog – even without paying – from humans that have breeding businesses, is support for their business even if it does not look like that at first glance. Breeders very often give away or sell for less a certain amount of cubs since they cannot predict the number of cubs that will come in a litter. Another reason can be a deadline, since after certain age it is more difficult to sell a cub. Except this, some cubs do not meet breed standards which lowers their price or makes them free of charge. Sometimes, breeders give them away due to some anomaly that they often keep to themselves. Also, they can give away the whole litter especially if it is the first litter of one bitch.