It allows the SimEngine to adjust your team's offensive tempo based on the situation in the game. In general, an adjustment will be made later in the game if your default tempo doesn't match a given situation.
For example, if you're an Up-Tempo team and have the lead late in the game, the SimEngine will switch you to Normal or Slow-Down to run out more clock on each possession. On the other end, if you're a Slow-Down team and are trailing, the Sim-Engine may increase your team's tempo to catch up.