Testosterone is not a supplement. It is a steroid hormone, that, like estrogen, is soluble in lipids. In particular, testosterone is both androgenic, and anabolic, and has a long list of benefits.
AIDS research has shown that test levels of 1000, or higher, provide much improved outcome for sick folks (HIV people), and remains the treatment of choice for many debilitating illnesses.
It improves mood, provides a sense of well being, lowers blood pressure, raises base metabolism, and lowers diseases of aging, improves fat distribution, and preserves lean muscle mass.
Nowadays, more and more, folks are getting T-therapy, which is a huge money maker, because like almost all American medicine the profit margins are over the top. They are also getting it because IT WORKS.
You'll want to read about this from qualified sources like reputable organizations such as the American Heart Association, American Cancer Society, and HIV agencies, geriatric and HIV studies. There's been LOTS of research done on this. Watch the movie "Bigger, Stronger, Faster" for a reality check about the potential ups and downs of AAS.
The GAO also produced a report for Congress in 2007.
Much of the information you've received here is ill qualified.
Many of the OTC stuff in nutrition stores are JUNK, and will cause problems like "bitch tits" because they bump your estrogen to bump your test. DUMB. You should stay away from that stuff. It's crap. Take the real thing.
If you engage in anabolic use, the main things to monitor closely are blood pressure and electrolyte levels. You can have serious health consequences from high bp, or low sodium / high sodium.
Remember: the highest mortality is from obesity. The second highest is from smoking. Both are FAR more dangerous than testosterone.
Like anything exogenous the affects go away when you discontinue use. The notion of "cycling" is absurd. Get on; stay on; watch the important stuff, or don't bother.
In America, AAS is big business, just like pot, with both sides having no interest in a truthful discussion nor in decriminalization because there is so much money involved, with the dangers grossly overstated. E.g. the cost of product is extremely low, but the medical and "fitness" community have made huge profits with their control of stuff that works, and...is junk.