Why test drugs?
As of 2020 only 26% of substances contained pure MDMA and rest either had various adulterants in them or no MDMA at all. (Source: drugsdata.org)
In an unregulated substance market, drug checking is important to determine if you're getting what you paid for, and to rule out any possible harmful or undesirable adulterants. It’s common for drugs such as LSD to be laced with NBOMe, or MDMA to contain PMA, both of which are harmful.
With the rise of new Research Chemicals/Designer Drugs aka NPS (New Psychoactive Substances), it’s common to see undesirable compounds laced with drugs. The risk of consumption of such substances is high. Test your drugs and always practice harm reduction!