there are many reasons they need your pay slips.

1st of all, calculate your PCB.
2nd, reference check (very important).
3rd, company policy. they might need to seek for approval to pay some1 x2 or x3 of their previous salary.

end of the day, take it or leave it. Every Company has their policies. I have gone to interview with 1 listed MNC as well, they even do a background check on my CTOS, SSM, directorship in other Company. Because at certain lvl, you need to be very 'clean' to them. i have heard a lot of bad stories on sh!t hit the fan when the HR did not do a reference check or back ground check.