Jesus said: 'Love the Lord your God with all your heart and with all your soul and with all your mind.' This is the first and greatest commandment. And the second is like it: 'Love your neighbor as yourself.'
Christians take 'Love thy neighbour' as an expression of what it means to be Christian very seriously. There's a common bond of humanity expressed in that, it's a kind of transcendent value for all times that stands up by itself.
So in terms of why people are doing this, that's the reason, it's an expression of love.
What do they want from you? Well Jesus said, 'I am the way, the truth and the life. No one comes to the Father except through me'. That's the Christian message ultimately, and is a pretty fundamental statement of belief (though liberal Christians have a very different slant on what it means compared to evangelicals), and that's the statement that new Christians are ultimately encouraged to accept. And why do they want you as a new recruit? It comes down to 'Love thy neighbour' again - expressed in the desire that you should receive the gift of salvation.
Bible knowledge, knowing that Noah begat Shem who begat Arphaxad who begat Cainan, is most definitely not the goal or endpoint - it's (usually) initially the acceptance of Jesus as your saviour, BUT the expression of Christian love for you is unconditional and NOT in anyway predicated on your unconditional belief, IYSWIM.
Basically they'll be very happy to have you along for the ride even if you don't formally 'sign up', you decline communion, etc., because there's an understanding that people might make the decision today, tomorrow, or in 50 years time, but they'd much rather you were there participating in the mean time....
Anyway like I said, Bible stories are all very well, but if, as it seems in your post, you see expressed in the church that universal expression of love, then that is IMO all you need to justify your participation.