A blessed medal is a sacramental, which the Catechism (CCC 1670) says “prepares us to receive grace and dispose us to cooperate with it.” It is basically an aid to prayer. The person who uses it should be using it in that spirit, as an aid in praying or as a help to developing their spirituality. If someone else chooses not to use a blessed medal, or doesn’t have one for any reason (can’t afford, is in prison with no access to one) then it is not necessary to prayer, any more than hanging a crucifix on the wall is necessary in order to meditate on Christ’s passion.
I would note that certain medals, such as the Miraculous Medal, are used for special purposes. Those who wear the Miraculous Medal because they believe in Our Lady’s apparition to and giving of the medal to St. Catherine Laboure are doing so in hopes of receiving the graces Our Lady said she would grant to medal wearers. Those who hang a St. Benedict medal in their home or carry one are hoping to gain protection against evil, etc. The medal should be seen as a help, much the same way that a comfortable pair of hiking shoes helps you hike better. People may still be able to hike in their sneakers or even barefoot, so the shoes are not essential but they can enhance the experience or help you get farther.
