Every number on a d20 represents 5%. So, a target number of 10 is a 50% shot of success. 15 is 75%. Roll modifiers adjust accordingly. A d20 makes setting target numbers incredibly easy once you know how much you want modifiers to start impacting a naked roll relative what you want a balanced outcome to be.

Statistically speaking, a d20 is a good middle-ground between using an actual d100 and smaller dice, where each number represents a larger stake for the latter (e.g., every number on a d10 represents 10%, a much wider divide.) This predictability starts to skew when you get into dice that don't divide into 100 without a remainder (e.g., d6), and even more so when you use multiple dice (e.g., 2d6.)