Brush mode (the Paint channel mode) is for editing the curve by hand. The default brush is the classic freehand curve - and since humans are bad at drawing smooth curves, the result is usually jagged. The roller brush tames it, which is also useful for cleaning unwanted grit out of hardware snapshots. Want grit instead? The spray, smudge, and texture brushes add different flavors of randomness for a broken or gritty sound.
The Brush tool also holds the magic wand commands that manipulate the whole curve: flip the polarity, flip horizontally, stretch the curve left or right (to fix - or add - asymmetry), and global smoothing.
