Well, there’s blessed and blessed. There are plenty of blessed objects which are okay to use in ordinary ways (like if you have your car blessed, or the way we ask for a blessing on our food by saying grace).
The blessing used on rosaries, crucifixes, and other religious objects is a more focused blessing that makes them into what we call “sacramentals.” Sacramentals are dedicated solely to religious use, and a person using them to pray can receive extra graces through that blessing. And that extra grace is why we get them blessed. Obviously there’s nothing stopping you from saying the Rosary on your fingers or on a knotted string, and it might be a good idea in certain circumstances. But all things being equal, sacramentals are a great spiritual aid. (And yes, of course demons hate sacramentals while most people like feeling blessed objects around.)
However, in return for God’s blessing on these objects set apart for Him and our relationship with Him, we ought to treat sacramentals with more respect than ordinary objects. If we have to get rid of sacramentals or if they break, we should dispose of them by burning, breaking, and/or burying them thoroughly, to prevent any person from using them with disrespect. And of course, really sacred articles that are consecrated and used at Mass, etc. are subject to similar rules, for similar reasons. They are set apart for God and can never again be used for ordinary purposes without insulting Him.