what ever set up u have,u do need to change water.

water changes is similar to FW,where u siphon out the old and replace with fresh SW.
just premix yr fresh SW beforehand...so as to ensure the salt has properly dissolved.oso ensure the pH,KH,salinity plus temperature matches yr tank's water.

water changes are the fundamentals of maintainig good nutrients export....may not be crucial in a fish only tank but for a reef tank,besides nutrients export,it also replenishes trace elements.