Well, I think even using the term "designer steroid" would be giving these people who sell these steroids, too much credit. First, they're not unknown androgens which have never been created previously (as was the case of THG, a true designer steroid), rather these guys are simply looking up exotic looking androgens in Vida's book, contacting the Chinese and sending them the structure. There's nothing "innovative" about it and requires zero knowledge. It's also ridiculous to attempt to sell them as "supplements" as they most certainly aren't naturally occurring. It's because of this that the FDA has recently begun to investigate these people selling these products as they're essentially selling unapproved drugs, not dietary supplements.
My problem however is that these are androgens that were never studied toxicologically to any extent. I think that much of the preaching over the years about the idea that anabolic steroids can be used safely (which they can) has led many to misunderstand the notion and believe to think that all androgens are these completely innocuous compounds, which they most certainly are not. The anabolic steroids which made it to the market within the U.S. or other countries were studied for selectivity prior to human use. That is, they've evaluated activity at the progesterone receptor, mineralcorticoid receptor, estrogen receptor and glucocorticoid receptor, to ensure unwanted adverse effects are not a major issue. I think people take this for granted and believe that any androgen is going to be selective and not have this problem. This is not unique only to the study of androgens, but steroidal aromatase inhibitors and really in any situation where the therapeutic compound is going to be a steroid or any compound that may cross-react with other receptors aside from that being targeted. Having a steroid which has activity at the MR for example, could have some serious adverse effects. Anecdotally, I've seen some reports in the past of people stating that a certain "designer steroid" had a diuretic effect. Well, I hope this doesn't come as a shock, but anabolic steroids should not be acting as a diuretic! Yet I've commonly seen people tell others that that's "normal" and nothing to worry about. Well, anabolic steroids do not act as diuretics, much the opposite in fact. If one were, that might be indicating that the steroid in question is acting as an MR antagonist. That's not something to be scoffed at either. I've also seen other reports that fit in with this...headaches, lethargy, drowsiness, nausea, vomiting and it could easily be worse, in some cases resulting in very serious adverse effects, potentially cardiac arrhythmia.
Then, and I don't know which is worse, you also have these people selling progestins, not androgens, on the most ridiculous and incorrect reasoning I've come across. They claim that nandrolone, trenbolone, oxymetholone, and so forth are "progestins" and hence their reasoning is sound. Unfortunately, not one of those steroids are progestins, not structurally nor pharmacologically, rather they are ANDROGENS. Yes, some may coincidentally have some weak progestational activity, but this has nothing to do with their anabolic activity. Progestins have been used to treat cachexia (due to increasing appetite) but the "mass" gained is all fat mass with a small amount of edema. Don't take my word for it, search for studies on megestrol as that's FDA approved for treatment of AIDS related cachexia. One could also look at the effects of medroxyprogesterone, another progestin, though it's used as a contraceptive (Provera) and see that it again does not have beneficial effects upon body composition. To add further to that, one is taking on a number of risks using progestins. Thromboembolism, suppressed endogenous testosterone production, decreased libido, gynecomastia, edema and hypertension as well as negative changes in blood lipids, etc.
The only potential benefit, and the reason why one would gain weight (albeit all fat mass) is that they may stimulate appetite. So in summary, the benefits are that you can increase appetite, produce a gain in weight that is almost exclusively fat mass, and suffer those potential adverse effects I discussed previously.