It's to attract more casual gamers and widen the potential audience. Few people want a game they can't beat and cause them nothing but frustration. Few people want to spend hours and hours mastering a game.
Hardcore gamers might not like it, but that's why different difficulty levels are provided. The real frustration for the hardcore comes in when one either can't choose a higher difficulty at the beginning (Diablo 3... normal was a waste of time), or cheap options to win are provided even in harder difficulty settings (pause+potion spam in Ys Celceta). The latter can be partially solved by simply imposing limits on oneself for a personal challenge, although it still cheapens the feeling of victory since clearing hard modes is no longer for only the top few percent.
