LogFAQs > #939893256

It's a tradition where it was intended for young audiences to yell the attack names with the character. The tradition began with Mazinger Z, which is considered the first Super Robot anime. The producers of the show felt that if the main character, Kouji Kabuto, shouted out the attack names each time the mecha did them, then it would give the target viewers, who were aged around 3 to 10 at the time, the ability to literally join along in the fun.

The logic behind this is if the kids were to directly interact with a show they already liked, then they'd like it even more and would stick with the show in the long run. Needless to say, the strategy worked, and literally all other mecha anime in the 70's (sans First Gundam in late, late '79) copied the trend.

Thus, the tradition was born and still anime shows, regardless of whether they are mecha themed or not, use it, regardless of their target audience.