OK, I'm going to use one of my patented "weird" analogies on this one. Hold your hat, as this one is a bit "out there".
Ok, energy isn't really "dirty" or "clean", what *I* mean by "cleaning" energy is to remove the things that are disrupting to my energy, while keeping the energy itself for use. Here comes the weirdness.....
Think of the energy itself like a pair of socks. After they have been worn a bit, they have bits of the wearer and what ever they have been walking through embedded in the fabric. So, for someone else to wear them in a comfortable and healthy way, they should be cleaned before the other person wears them, right? This gets rid of the dirt and the first person's odor and such. To clean socks, you wash them in water with soap. To "clean" energy, you kind of do the same thing by either filtering it through your shields (advanced technique), or by passing it through another energy field, preferably an elemental one you are compatible with. (basic technique)
All right, before we go further, I'd like to explain more about what can attach itself to basic energy. Energy is what carries and powers such things as "spells", for one thing. It can also carry the "flavor" of your personality, emotion, and/or any personal energy "tangles" you are currently experiencing. We are beings of energy, with a material component.
Physics tells us that what we commonly think of as "solid" and "material" really isn't. On the atomic level, "solid" things are made up of mostly "empty" space, with atoms and molecules held in tight energy bonds and fields. On the subatomic level, there is no such thing as "solid" matter, because ultimately, all things at that level are made up of coherent energy fields, period.
So going from both *my* personal belief set, and current scientific thought, it is NOT unreasonable that when energy passes between two things, people, or areas, it may be carrying "inclusions" of organized energy in the form of emotional, or personal, or directed energies. For "ordinary" people, those NOT sensitive to energy and/or its workings, this poses no issues, they neither perceive it, nor are they generally greatly affected by it. There are exceptions, of course. For instance, the effect of a charismatic speaker is an effect of projected energy just as much as it is an effect of well-chosen words. There are other exceptions as well, of course, but I think you understand what I'm getting at.
These inclusions of undesirable energy can affect one if they utilize the energy in it's "raw" state, i.e., without removing those things that are detrimental to the user. This is one of the things that shielding for empaths and others strives to help with--the accidental or unavoidable absorption of unwanted energies like emotion. For those that gather or use energies for working, cleaning any outside energy you gather or use is a VERY good idea.
As far as personalizing energy goes, unless you exercise precise care with energy you send forth from yourself, any energy you have touched or used has acquired "inclusions" of your own energy, emotions, or personality, and IS personalized to you. If it was not cleaned first, it also continues to hold most of it's previous inclusions as well.....whatever you did not get "brushed off" into your own energy.
Ok, I know you must be thinking "right, I get that energy can hold stuff from where it's been and done, but so what? What does that mean TO ME?" Well, if you take incompatible or detrimental energies into yourself a number of things CAN happen. You can get ill more easily, you can become stressed, confused, or lose your true sense of self. You can become obsessed with the source of the energies you are using, or you can take on personality traits alien to you. It is also very possible to not be aware of these changes in yourself for a long time, or never see it at all.
So, if you have followed me so far, you can see why you would want to clean any outside energy before you use it in most situations. The next question of course is HOW do you clean energy? The easiest way to clean energy is the same way you would go about grounding your own energy. Most people seem to find it easiest to pass energy in a sort of loop through the earth and back into themselves to do this, while envisioning calm, purifying thoughts and feelings. Some people do this through visualizations, while others use feelings or a tactile sensation. Whatever works best for you.
I hope this helps in understanding the things you have been researching. These are just *my* thoughts and opinions, and not to be taken as anything other than that. I don't think there is any "one" "right" answer. I think there are many ways to look at anything, so take from this whatever works best for you.
Black Rose