why bank statement needs for tourist visa

It's OK your friend send you the invitation letter to you by himself. BTW, it should be in Chinese.
They ask for the bank statement, for they need to know you have enough money to support yourself while in China. The balance usually equals to USD100 per day. The longer you'll stay in China, the more balance you should have on your bank account.

bought a 5 days/4 nights package tour from the travel agent... hotel/round trip airfare are all paid except meals and maybe some souvenirs, is bank statement still needed... if so, how much would be the our bank balance.
thank you

trevor, you may ask the travel agency to provide you an Invitation Letter, then you don't need to provide any bank statement.