What does 'し' mean in these scenarios

Question

何しに原宿に来ましたか

I understand what the sentence generally means, but I am unclear what し means in the sentence above. Is the し just a conjugation of する, and if so, when should I use し instead of する? Can なんで原宿に来ましたか be used instead without changing the meaning?

Another example:

日本語ができると旅行がしやすくなって便利だと思いますよ

Same as the first example, I understand what the sentence means, but don't understand why し is used. Doesn't removing the し to become 日本語ができると旅行がやすくなって便利だと思いますよ mean the same thing?

Answer 1

Both these し come from する but they are used differently.

For your first sentence,

何しに原宿に来ましたか。

the pattern is:

masu stem of verb + に + motion verb

This translates as "do motion verb with the purpose of doing main verb", e.g. 食べに行く (go to eat), 見にくる (come to watch), 何しに来ましたか (What did you come to do?).

Replacing し with する would be ungrammatical.

For your second sentence,

日本語ができると旅行がしやすくなって便利だと思いますよ。

the pattern is:

masu stem of verb + やすい

This translates as "easy to do verb".

So 旅行がしやすい is "easy to make a trip". Again, replacing し with する would be ungrammatical.