This plant is part of a bigger chain. So while yes, on its own it seems waste of effort, as part of the entire chain it’s a reasonable step to be more environmentally friendly and recover some energy in the process.

A local plant desalinates water, resulting in fresh water and a brine solution that has much higher concentration of salt in it than regular sea water.

Dumping the brine solution on its own would kill most plant and animal life around the dump site due to large saltwater concentration, so an alternative method must be found to dispose of the brine.

Waste water from other processes can be mixed with the brine to bring it more in line with seawater salinity, making it safe to reintroduce to the ocean without severe ecological impact. This waste water is deemed to difficult or intensive to purify and treat to bring it back up clean water standards, and I’m assuming tested or filtered so as not to introduce hazardous chemicals that could damage the reverse osmosis membranes as well as sea life.

Because there is way to mix the waste water and brine through membranes that can be used to generate electricity, this process is utilized to recover some of the energy expended in purifing the original batch of seawater resulting in the brine.

It’s not a perfect process but it is a means of getting some use out a waste product, similar to burning garbage or rotting food rather than just dumping it into a pit and letting it rot and release methane.