Weeds are bullies. They strangle their neighbors. They steal water from purposely planted perennials. They fight for space. They fight for nutrients. The more space and time they are given to establish themselves, the harder they are to combat.
Manual weeding on a regular basis is a needed step of garden care. Gardens that use purposeful groundcovers to fight weeds for space require less and less manual weeding as the groundcovers establish themselves, but the need for manual weeding never entirely ends.