Ask Google about "difference between industrial and craft union".

There's US labor history involved. The AFL-CIO was formed by a 1955 merger of the American Federation of Labor and the Congress of Industrial Organizations. The AFL was the umbrella organization for the craft unions - electricians, plumbers, etc. The CIO was the umbrella organization for the industrial unions - everybody non-management in an auto plant, everybody non-management in a steel mill, etc. That's what "wall to wall" means.

Agreeing on the bargaining unit is a major issue in employer-union relations. A "wall to wall" agreement avoids internal issues over who can do what job. (Is plumbing for compressed air a plumber or a steamfitter job?) That helps the employer. But it gives the union more leverage over the employer because the union has all the employees.