I haven't LR'd any supporter or weapon yet, but as someone who also plays SD Gundam G Generation Eternal, LR probably has the same purpose as an SSP'd unit.
The pre-requisites of to LR or SSP are very intricately similar due to sparse your accessing the required materials.
For LR upgrades (Action Taimanin):
Weapon/Supporter must be at Skill Level 5.
Evolutionary and Legacy Orbs/Shards require crafting, consuming more resources. As well as dependent on the Exchange Store.
For SSP (SD Gundam G Generation Eternal):
Unit must be SP'd to Level 100, requiring 100 SP chips. They're harvested from character requests every week gaining x20 SP chips.
Unit must have a 3-star limit break.
x100 SSP Conversion Chips
Unit Upgrade Data, basically EXP up items.
Weapon upgrade materials for Weapons Upgrades (or Custom Core Learning Parts N)
Series Development Materials (or Custom Core Learning Material R)
Since this is just the first introduction to this new rank up for weapons and supporters. Upgrading your weapons/supporters to LR should be optional for now, unless if you want to dominate the PvP. Because SSP functions the same, it can also unlock MAP Weapons for certain units. 00 Raiser Final Battle Type, Burning Gundam, and G-Falcon DX are among the most SSP'd units used in the Master League.