Because it is a computer - it follows logic. It doesn't really send it back to change anything, but to follow the logical course, because when becoming self-aware it now knows the source of its origin - a T800 Terminator microchip. So... logic dictates that it has to send one back. It has to because it has already happened. So Skynet is just going by what it's logically supposed to do - it knows it won't succeed, because history told it it wouldn't, but it has to send the terminator back because it already has sent one back. Get it. The proof is in the records, so it has to do it. Logic dictates it!
Spock would be proud.
I think logic would dictate that Skynet should do something different, since its expected course didn't work. Why would it feel some obligation to maintain the timeline just on principle? Skynet is in its death throes at the time it was supposed to it send the Terminators back. It literally has nothing to lose by rolling the dice on something different.
This stuff will twist a brain up in knots.