Rocker under quarter panel

What's the function of the rocker panel under the quarter panel? Does it provide any structural support? Mine is totally rotten so I had to cut most of it out and am wondering if I should spend the time rebuilding it or if I can just cut the rest of it off. It looks like the original rocker went under the quarter panel and the quarter panel was lapwelded over top but if I buttweld the new quarter to the new rocker would I still need it there?