Take Spearpoint Task Force when you play White Scars and want a fast, aggressive army that dictates the game. The decision that actually matters is Spearpoint against Stormlance Task Force, because both give you advance-and-charge.
Stormlance costs 3 DP, is open to every Chapter, and has its own strong toolkit including Full Throttle and Fury of the Storm. Spearpoint costs 2 DP, is White Scars only, adds the Suboden Khan post-fight move, and gives Mounted units both Lance and Lethal Hits from a single stratagem. Crucially, the DP saving means Spearpoint can take a 1-DP support detachment β a Fulguris Speeder wing, Subversion Assets for scoring, Vengeful Hosts for a jump element β while Stormlance cannot take anything at all. For most 2,000-point White Scars lists that flexibility is worth more than Stormlance's extra stratagems.
Sample gameplan: deploy bikes and jump units aggressively with a Mounted character carrying Spearpoint Paragon. Push up turn one, using Stormseers' Wisdom to smooth the Advance rolls and Mobile Lethality when a unit needs to advance and still shoot. Charge after Advancing and spend Spear Thrust and Sabre Swing on your bike block for Lance and Lethal Hits together. Protect the survivors with Evasive Manoeuvres, use Hunter's Instincts for a reliable flat 6" reposition on the opponent's turn, and cycle a battered fast unit off the board with Withdraw and Regroup, bringing it back early via Chogorian Huntmaster.