IF you'll look, you'll also see it on conc hgwys, airport aprons, & any place you'd find conc slabs,,, its joint seal & the purpose is to keep incompressibles OUT of the space between the slabs,,, why you ask ? when conc ( or any solid ) gets hot, it expands [ hi-skoul fizzix ] when it expands, the ' gap ' gets smallered,,, when it smallers, the stuff that won't compress causes the conc to ' spall ' ( fracture [ break ] )
joint sealants are designed for optimal performance at certain depth:width ratios,,, to seal the jnt w/more would cause either adhesive, cohesive sealant failures OR, at the least, shorter svce life.
underneath the sealant you'll usually find a foam-stuff backer rod installed @ a depth to prevent oversealing,,, simple, huh ?
if you hear it from a guy in the apron store, be VERY suspicious the mtl/method will work,,, when it time to build something together, they won't answer phones NOR help