I get it, but
1.) I'm not one of those who carries a huge backlog; I usually buy the game I want to play and complete it before starting anything else. If said game merits subsequent playthroughs and intrigues me enough, I'm content to run it back as necessary. With Souls game (well, only DS1 at this point,) conquering the beast was enough of a rush to make me want to try it again with experience under my belt and do it better, i.e.: beating personal bests or perhaps trying out a playstyle that I hadn't tried the first time just to get a feel for everything the initial playthrough showed me could be appreciated but went unexplored.
2.) Subsequent playthroughs don't typically take nearly as long as the first. It took me dozens of hours to beat DS1 my first time, but after "getting" it, the next run, I cut my playtime by over half, and subsequent runs by half of that. Not saying anyone "needs" to play games multiple times, but in some cases, one playthrough is a single menu selection from restaurant that offers hundreds of items; yeah, you ate there, but can you truly say you "know" it well enough to pass judgement on its overall value?
If you or anyone is content having completed the game, i.e.: beaten the last boss and saw credits roll, and that was enough, then I can't tell you to feel otherwise, but as someone who's played some games multiple times and had as many differing experiences, I can attest that sometimes, it's worth it. My INT build in DS1 was an eye opener; knocking out a third of a boss' health bar with a single cast was SO satisfying after struggling through a dozen deaths over the same boss with my melee builds. Was it overall vastly different? No, but knowing "how" to best the game after playing it enough times is in and of itself a feeling of accomplishment that made those additional playthroughs worth it.