It's not necessary they do so in order to achieve their goal of winning another NBA Championship. They only won 52 games in the regular season last year. Neither of the prohibitive favorites for the best record in the East (Cleveland and Chicago) would definitely be favored in a seven game series against the Heat (presuming both Shaq and Wade to be reasonably healthy). If Shaq and Wade aren't both healthy, none of this matters.
Still, 50 regular season wins are far from assured as the Heat seem intent on making things as difficult as possible for themselves this season. In addition to the reasonably assumed absences of Shaq and Mourning from 15 games each (minimum), and Wade from ten (he's getting an MRI on his right hand soon), Jason Williams postponed his knee surgery long enough that he will be out until at least mid-November, Pat Riley did not sign anybody of note this summer to back up James Posey or the rapidly disintegrating Williams/Gary Payton tandem, and Riley is apparently counting on Dorrell Wright (career minutes played: 160) to play productive minutes at all three perimeter positions depending on the circumstances at leastr until the remains of Shandon Anderson are healthy enough to pass a physical (see Jason Williams story linked above).
With Cleveland, Chicago, Boston, and Orlando figuring to gobble up most of the wins that Detroit and New Jersey give back to the field this season and New York, Atlanta, Charlotte, and Toronto each possessing reasonable hopes of cracking the 30-win mark, Miami, despite having two of the top ten players in the entire league, risks finding themselves amidst a tight six-team fight over the third through eighth seeds in the East. All the question marks that hover over Miami's regular season campaign are more likely to create post-season complications if they attempt to win three or four successive series without home-court advantage.
Most of the time, teams make gradual steps toward a Finals appearance. This off-season, Miami and Detroit have not just allowed Cleveland and Chicago to build teams that look better suited for the long haul of 82 games, but have failed to supplement their outstanding frontline talent such that anything that deviates unhappily from the organizations' respective plans will open the door for an Eastern Conference team to make a tremendous leap forward and earn a shot against whoever is left standing from the Western Conference.