Laver comes along with onigiri and a sheet of dried laver has been generally spread from the Genroku era (the end of 17th century) in the name of such as 'Asakusa dried laver,' nutritious and convenient because cooked rice does not stick to the hands, which established a relationship of laver and onigiri.
おにぎりと言えば海苔だが、板海苔が「浅草海苔」などの名で一般にも普及したのは元禄の頃よりで、栄養もあり、手にごはんがくっ付かない便利さも相まっておにぎりと海苔の関係が出来た。