Same reason you would use Synthol. Probably plenty of pros using this, but no one will admit to it for the same reason most lie about the Synthol.
My only problem with either of these is that it doesn't look like real muscle. Yeah I know many will say SEO will not be visually detectable if done "right" but I simply do not agree. I haven't seen a single SEO before and after comparison where you couldn't notice it. Probably same with PMMA, and Rich said something about the bicep not looking like before the teatment, the peak changed. Look at some of his comp pics on MD and tell me if it's an improvement.
If Phil Heath uses these things in his arms, then I might change my view. because his arm shape looks good. Of course "everyone" on here said Phil would never use SEO, because his arms were huge from the start of his career.