Mixing Strategy and Randomness allows for more varied and exciting gameplay. On hand a game of pure luck is boring and technically no one wins except for one grinning tool who did nothing to earn what he got, kind of like life, and on the other a game of pure skill no matter how complicated has a "correct answer", there is a perfect way to play, the mathematically most efficient move for any scenario can be determined and if you and your opponent know them it will be boring. So the key is to strike that balance between them, which as I have previously mentioned is all preference, there is no right or wrong way to go about it, just avoid making a game exclusively about one or the other.
Randomness doesn't need to be on a linear scale of values that equate advantage to disadvantage like flipping a coin, rolling a dice or drawing a card, it can be much more uncertain or random in how this aspect of luck can effect you. It could be the layout of the game board or map for instance which will effect what strategies you should use going ahead and also provide you with a new scenario so even if there is a "correct answer" no one is going to know it if the boards layout is one in a hundred.
There are methods randomness that can be applied to a game that doesn't involve a strict good or bad connotation to and are just modifiers to the experience to suggest players alter their strategies to allow them to gain the upper hand from their own merit, which brings the pride in victory from a skill based game, but also brings the tension, excitement and anxiety from a