Many reasons! Here are a few:

• Understand technical materials better and faster. (Primary)
• Not missing out English conversations in my country.
• Defending myself in English arguments.

I wish my English were as good as my Mandarin. Have been practicing on Reddit for more than a year and have improved a lot since then! But still lack some basic language sense and find it challenging to express abstract concepts precisely and concisely.

ps: hmm... lemmy.world server kind of unstable