My personal journey with this...
When I was a fairly new practitioner, and knew absolutely no Tibetan, it was suggested to practice using the transliteration, to come as close as possible to the sound of the original Tibetan words. The rationale, as many here have pointed out, was the power and blessing that those words carried.
I didn't really believe that rationale at the time, but out of respect for my teacher, I was willing to suspend judgment and follow the suggestion.
Later, I learned enough Tibetan to be able to read my main practices directly (albeit slowly) from the Tibetan script. During a period of longer retreat, I had some experience of the blessing. It seemed to me that somehow the sound and imagery of the Tibetan words themselves conveyed the enlightened qualities of the author's mind, so that I actually could feel something of that presence and power as I read them and chanted.
Interestingly, I learned that the English version of that text, which had been translated by a committee, underwent multiple published revisions as the translation committee consulted additional teachers and gained more nuanced understanding of the original Tibetan. As a practitioner, it would have felt a bit strange to have chanted one English translation for a number of years, and then switch to different English text. By using the Tibetan directly, however, I could integrate the additional levels of meaning conveyed by the revised translations without changing what I actually chanted.
All that said, however, it is crucial to understand the meaning of the text, and the purpose of the liturgies, in order to really benefit. It takes time and some effort at study to learn a liturgy, just as it takes time and practice for a musician to learn a complex musical score. In the beginning it seems awkward, but with patience and practice, one gains a sense of flow, ease and enjoyment.