Spray buffering is when you start holding the spray button before you are in range of any sprayable objects. When done correctly, you will start spraying as soon as you go within range of a tag, removing the need to react quickly. Although, because spraying works by ticks, you won't necessarily spray the tag at the earliest possible frame. For a more detailed explanation of the implications of this mechanic, see Secret of the Dragon.
For normal tags, buffering will consume one extra can (one for the buffer, another for the tag). However this is not true for unique objects such as switches and timers - if you buffer these, tagging them will not consume a can, resulting in only 1 can used.
Other benefits include the graffiti cam not activating when spraying certain tags. The tags for which this occurs seem to be random, and it doesn't happen every time. Because of this, buffering even trivial tags could be desirable.