I have no idea why this thing caught on, but I think it’s a bit strange. I’m not a big believer in any kind of special mystical powers of any particular prayer, and in general think that the current schedule of prayers is a bit too heavy, and should be trimmed back. I think it would be nice to have a ‘Shlah Awareness Day’, and I guess that 29 Iyar is as good as any, though it’s a bit tough coming right after Yom Yerushalayim / Shmuel Hanavi Awareness Day (28th of Iyar is ostensibly his yahrzeit and is observed by many as a minor holiday; thousands make a pilgrimage to Nebi Samwil, his ostensible tomb). I’m just weirded out by the sudden massive popularity that this prayer gained. I have no good explanation.