Louder sounds cover up softer ones, especially those that are nearby in frequency, which is called *frequency masking*. To make the maximum number of different sounds audible at one time, it is useful for them to be spread out across frequency in order to minimize the masking of one sound by another.
At low levels, frequency masking is contained in a relatively narrow frequency range around the masker. The higher the level, the more effective it becomes at masking other frequencies.
Frequency masking is also used to deal with differences in ambience or room tone in the sound between cuts. A track of background noise will run constantly behind all of the edits to mask the differences of the production sound changes, and make edits smoother.