There are no benefits for an average person in China to join the party aside from your member card doubling as a free public transit ticket.
First, you can't meaningfully participate in the political process in any capacity, without already "being somebody" in the party.
Second, only 1 in 1000 of party members will ever see any chance to work in any meaningful position. Yes, the actual amount of people who do politics in China is no more than ~70-80 thousand extremely privileged people, picked among the most fanatical candidates.
Third, even if you already "is somebody," and passed the impossible odds, and be picked out from the most fanatical party members, your remit will be no more than to blindly execute edicts passed from above.
Fourth, if you survived for over 2 decades in a position of low ranked official, your brains will not. Anybody who spent so much time mentally tortured by routine work and doing meaningless, brain dead "ideological research" will have their spark extinguished, no matter how talented they were.
Fifth, when you turn a grey haired party veteran who can recite all Marx works from memory, and have a genuine chance getting to provincial or national level, you are a useless husk of a person, who can't do anything but mechanically follow the politburo.
Sixth, if you survived even that, and surpassed everybody else at doing that ritual, you end up becoming a man in ranks of Xi Jinpings.
Jiang Zemin, was the only man ever known to "hack" that system, and he is universally hated by party for that.