Premining devalues future work done on a cryptocurrency because the underlying idea is to decentralize money, and the coins found at the start of a blockchain are easier to generate because the target difficulty for finding a block is significantly lower.
The most famous case of premining I can think of is https://en.wikipedia.org/wiki/Coinye (ltc clone), where people started mining it after dogecoin began. Dogecoin was in its first few weeks of release and had an extremely active number of miners; and was even more profitable to be mining than bitcoin at the time. There was speculation that having a celeb figure attached to a currency could bring the cryptocurrency movement mainstream; but when the block explorer for kanyecoin was released it was found that a very large number were premined by the developers, and few people considered it was worth dedicating mining time towards. The devs basically dumped all of their coins onto an exchange and abandoned the currency soon after. Premining is bad for building up a community of miners towards which an altcoin can be established for both trust and future earnings.
The most famous case of premining I can think of is https://en.wikipedia.org/wiki/Coinye (ltc clone), where people started mining it after dogecoin began. Dogecoin was in its first few weeks of release and had an extremely active number of miners; and was even more profitable to be mining than bitcoin at the time. There was speculation that having a celeb figure attached to a currency could bring the cryptocurrency movement mainstream; but when the block explorer for kanyecoin was released it was found that a very large number were premined by the developers, and few people considered it was worth dedicating mining time towards. The devs basically dumped all of their coins onto an exchange and abandoned the currency soon after. Premining is bad for building up a community of miners towards which an altcoin can be established for both trust and future earnings.