I've worked with a few really good designers. They can add a lot to a product when designers, PMs, and engineers are all equal stakeholders who can contribute

I've worked with a few really good designers. They can add a lot to a product when designers, PMs, and engineers are all equal stakeholders who can contribute ideas and point out shortcomings. E.g. a good designer can take complex functionality and create the optimal interaction pattern for that functionality, and make it look and feel good to use, without sacrificing functionality.