If you want to join the party or work in a government department, you'd better join the league. If you don't plan to be a civil servant in the future, you don't need to join the league.

China * * * Communist Youth League (hereinafter referred to as * * * Communist Youth League) is an advanced youth organization led by China * * * communist party, a school for young people to learn Socialism with Chinese characteristics and capitalism in practice, and an assistant and reserve army of the Party. Joining the League is a necessary condition for joining the Party. If you want to choose a job smoothly after graduation and want to develop in the civil service industry, it is recommended to join the league as soon as possible and actively join the party.

Joining the league won't give you any benefits in the short term. Joining the league will give students an advantage, but in high school, everyone is a league member, and this advantage is very small.