'It's Evolving!"

[Katipunero]

So kids will get used to the concept that pokemon is also about evolution.

 

[The Seer]

Kids are just so forgetful that they need to be reminded once in a while.

 

[MizuJosh]

For new viewers and/or people who do not understand the basic mechanics of the series.

It's almost like when Ash came across a Heracross with some kid and was all "Whoa, is that a Heracross?"

 

[GalladeX]

Evolving can look like an attack, like Bide. It's always nice to know.

Yeah.
"It-it's evolving?"
"No dumb***, it's using Bide"

It's like when someone has a kid. Instead of saying "It's a boy/girl" you say it's "It's evolving". It's also to let noobies know what's happening.

It's almost like when Ash came across a Heracross with some kid and was all "Whoa, is that a Heracross?"

That could be like him saying that it's cool.
Like, if I saw someone with a moped, I'd go "Whoa, is that a moped?".

 

[JJDalts]

I'm probably wrong about this, but I think one of the only times when everyone wasn't shocked by a pokemon evolving was Paul when his Elekid evolved.

 

[Typhlosion Trainer]

Its probably for those who are new to the show I guess.

 

[Master_of_grass]

no the annoying thing is when ash says"whats happening". well pokemon master no wonder paul thinks your an idiot. after 4 regions of this you still ask whats happening when a pokemon starts glowing. IDIOT