Soft Plastic Trailers are important to increase:

• Buoyance: Soft plastic trailer increases the buoyancy of horizontal presentations keeping it higher in the water column – strike zone.
• Profile: Soft plastic trailers increase the overall size of the lure, subtly for smaller applications or very large for example in low lure visibility scenarios.
• Lure Action: Some soft plastic trailers can provide a subtle enhancement to the lure action, for example a soft plastic jerkbait, where in some cases the soft plastic trailer can provide most of the lure action, for example a creature soft plastic or a lizard soft plastic.
• Weight: Soft plastics trailers increase the weight of the lure which can increase cast distances and potentially impact rod (power and action) and fishing line selection.
• Mouth Feel: Some suggest that adding a soft plastic trailer will make the lure feel more natural when the bass bites such that they will hold on longer prior to spitting it out. 
• Scent and Taste: In slow fishing applications, some suggest that the soft plastic trailer can add scent and taste to the lure.