Why does the TAF provide forecasts at irregular intervals?
I assume you're referring to the FM (FROM) groups. The TAF itself is published at regular intervals but sometimes the conditions are expected to change significantly in between those times; obviously, the weather doesn't operate on a neat schedule. So the TAF will show the expected time of those changes.

According to the National Weather Service:


  FROM Group: ie. (FM1600)
  
  The FM group is used when a rapid change, usually occuring in less
  than one hour, in prevailing conditions is expected. Typically, a
  rapid change of prevailing conditions to more or less a completely new
  set of prevailing conditions is associated with a synoptic feature
  passing through the terminal area (cold or warm frontal passage).
  Appended to the FM indicator is the four-digit hour and minute the
  change is expected to begin and continues until the next change group
  or until the end of the current forecast.
  
  A FM group will mark the beginning of a new line in a TAF report. Each
  FM group contains all the required elements -- wind, visibility,
  weather, and sky condition. Weather will be omitted in FM groups when
  it is not significant to aviation. FM groups will not include the
  contraction NSW.