Heya Leyha
If you're upgrading from a lower quality PSU to a higher one, there's several benefits.
One of them is that the PSU won't draw as much power from the wall, to actually generate the power for the PC. With that I mean a 1000w 80+ bronze PSU would need to draw up 1250w from the wall, since it's at least 82% efficient. Gold on the other hand would draw up to ~1150w instead. Which means the electric bill would be a little less
Generally you'd also like to get a higher quality PSU, since the lower ones might not even be able to spit out their advertised wattage. fx. a quality PSU such as the EVGA Supernova 750w g2 is able to output ~749w under normal circumstances. A Raidmax Cobrapower 500w on the other hand will be able to output ~385w instead.
Higher quality PSU's also generally last longer, since they're not built cheaply. If you'l like to see a list over good PSU's check the PSU Tier list here on Tom's
http://www.tomshardware.co.uk/forum/id-2547993/psu-tier-list.html
Cheers