What does 'し' mean in these scenarios


> **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?




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.

