Hisotorically, the motivation was to permit spending of both the “native” or “distant” dedication anchors, with out introducing additional extra pin vectors. It had many short-comings, together with lack of efficacy below adversarial situations, and nonetheless costing substantial vbytes below even benign situations.
With respect to the number of pinning vectors and their historic significance, I do not assume I will do lots higher than BIP431 motivation part for TRUC transactions: https://github.com/bitcoin/bips/blob/grasp/bip-0431.mediawiki#motivation
In brief, there are a selection of pinning vectors that existed when the Lightning Community was first rolled out. “Full rbf” was not adopted throughout the community on the time. RBF charge pinning, bundle restrict pinning, lack of bundle relay, lack of bundle RBF, and so forth additionally existed, making replacements too costly (and incentive incompatible) or outright inconceivable given sure situations.
Since Bitcoin Core 28.0 a lot of this surroundings has modified: https://bitcoincore.org/en/releases/28.0/
With the deliberate replace to “zero-fee dedication” channels (https://github.com/lightning/bolts/pull/1228), these issues are considerably lowered or eradicated, leads to larger composibility, and leads to smaller transactions in vbytes. There’ll doubtless be a single anchor output that’s 0-value (or the trimmed HTLC quantity), and PayToAnchor making it free to spend by both counter-party and not using a signature.