Almalieque, once one understands the purpose of life, they can understand the reason behind using substances. This is because the purpose of using drugs of any form, whether legal or illegal, is to fulfill the purpose of life. Life itself has no definite purpose, it is a constant flow of action and feeling melding into memory, memory being the only thing one has remaining of their life at any given time. Until eventually one dies, and then their memories are lost with their life and nothing that they did during that life matters anymore (to them at least). So, therefore, the only time anything matters to someone is during the period that they are alive, which means that during this period the only purpose to their life is to experience everything that they possibly can given their circumstances until their ultimate end.
What I am trying to say, in other words, is that in a life where death is inevitable, the point would rather be to experience everything and anything this world, or perhaps even other worlds have to offer if it becomes a possibility to you. Drugs and substances included, a long with countless other things and combo-nations of things that life has to offer (ie: jet skiing, snowboarding, sex, sports, internet, games, concerts, foods, travel, etc). The more you do, the more life you've lived, and once you die there's no chance of coming back and trying again. Nothing ever happens exactly the same twice, and if you miss out on it the first time it's just another experience lost that you'll never have. It's best to just continue doing everything that you can all the time.
That being said, No I am not saying that it's good to use drugs or any substances regularly..but that is because you will be taking away from your sober experience of life by doing so which is also something to enjoy. It is also because many drugs do have harmful affects on the body and mind, therefore those should be done very cautiously and probably not frequently unless you would prefer to end your life sooner than expected. However, all drugs can be done cautiously and it does not hurt you so much to try them out maybe once or twice in your life, with a good balance it can be done and it will provide you with an experience that you would not have otherwise. If it's an experience that you would rather not have, that is your choice, however, that only means that you are taking something away and out of your own life experience as a whole. With that in mind, it's kind of ironic how people would consider using drugs "stupid" when in reality I would consider it quite the contrary.
Also, to many, drugs are used as a tool. This may not be the best reason for using them, but it's a tool for the modern world today nonetheless. Whether it is to relieve pain so that it makes your day easier to function, or to help fall asleep because you have to wake up early the next morning, or to help focus in school, or perhaps to even perform better sexually or to gain other's attention. These are all ways that drugs can be used in today's world, and although in some cases it might be considered silly reasons, the truth is that they work for those purposes and that is why people use them. Anything that can be used as a tool has a purpose.
Returning to my original point on life however, yes, I do believe that EVERYTHING that is possible SHOULD be experienced and done in a life time. I would go even to as far extremes as murder or bestiality, because those too are experiences that one could only have and truly understand if they commit to them, however, such extreme experiences are better to stay away from. Obviously because murder will truly harm another being, in fact it will completely terminate their life time which may not even be close to finished, or as fulfilled with experiences as yours might be for that matter (not like that makes it any more okay if it was). So, simply, certain things SHOULD NOT be done but mainly for the sake of others, while there really aren't many things that should not be done only for the sake of yourself. Bestiality is one of those things which I listed earlier as an extreme, because I still say that the fact remains if someone has had sex with an animal they have certainly experienced something in their life more than me, but there's just soooo many other things I would rather experience in life before doing that that it just doesn't seem like it would even fit on my list before my death. Not to mention it would be rather gross/disgusting and probably not very enjoyable hahah.
But like I said, those are the extremes...for the most part, however, life's purpose is to experience everything that you can no matter what that might be. And that is the reason for doing drugs.