Way back in 2008 I wrote about my theory of Financial Attrition:
Entrenched incumbents almost never lose, barring the occasional "dead woman or live boy" scandal. Such Untouchables have large campaign war chests at their disposal, have first dibs at party and caucus money, and often represent safe districts. The unlikelihood of their defeat acts as a deterrent force against any potential challengers, or, if some fool does step in, the challenger's party lifts nary a finger in assistance.
What do these unchallenged incumbents do with their money? They give some of it away to help candidates elsewhere. By helping push their marginal fellow partisans over the finish line they accumulate favors for future battles.
Oppositely, what does an incumbent do when he as a challenger? He empties his coffers! Electoral history is littered with the corpses of challengers who were outspent 3-to-1 yet were only defeated by a few percentage points.
When we find ourselves with uniquely qualified challengers such as Sean Bielat giving some old warhorse like Barney Frank a run for his money, we should understand that Bielat's use of funding will be extremely efficient at denying other Democrats around the country their precious funding. One dollar given to Bielat destroys several Democrat dollars as they scramble to play defense.