If you farm as SR, you’re likely to use your basic sub-weapon attack a lot, with good gear you can easily just run around holding auto-attack than to keep casting skills and having to position yourself accordingly. With its special attribute, Limacon gets a ricochet effect that hits adjacent enemies, increasing effectiveness.
Since you mentioned you’re following a build, I assume you are new to SR. Limacon is important because you can also use it alongside Retreat Shot. Most players that I helped in-game didn’t realize that or never knew how Limacon worked to begin with. With Limacon active, you cast Retreat Shot and then hold the sub-weapon attack (Default is C), doubling your damage as long as you have a target to shoot while Retreat Shot is automatically shooting as well. You can compare this combo versus Marching Fire against bosses and see for yourself.