The main reason for spacers is that not all wheels are compatible with the brake and caliper kits, So the spokes of the wheels might hit/ scrape the caliper, so the spacer is used to push the wheel spoke away, thus not hitting the caliper. But make sure the spacer is not to big as that can result in the wheel hitting the arch or going past (Normally people roll there arches to allow there wheels to sit further out).
You can also get them for more of a visual for instance if your wheels do not sit as far out as you like.
In some cases yes it can improve the handling as the wheels as further apart it will improve weight distribution, but personally if they are the OEM wheels i would just stick to where the manufacture has them set.