The relics of the saints are a time-honored tradition that goes back at least (to my knowledge) to the martyrdom of the saints under the Roman emperors -- when St. Polycarp was burned at the stake around A.D. 165, his followers gathered up his bones as "more precious than gold or precious stones". Fast forward to King Charles I's execution at Whitehall -- bystanders blotted up his blood with handkerchiefs and preserved them. Certain High Anglican quarters revere him as King Charles the Martyr, and miracles have been attributed to his relics, but he's never been canonized in the Catholic church. :-D Then of course there are all the Victorians with their locks of the late lamented's hair woven into mourning brooches, etc.
Point is, souvenirs, even souvenirs that moderns see as rather grisly, are focal points for devotion, affection, and even prayers and pleas for help. Catholic doctrine does not attribute any quasi-magical powers to the relics themselves. Rather, the bodies of the saints are temples of the Holy Spirit and to be bodily resurrected on the Last Day, so they are worthy of honor and respect -- but any miracles attributed to them are due to the Holy Spirit working by and through means of the saint.
After all, God did not simply will redemption, which he could have done purely through divine power without any physical manifestation. Instead, since humans are both body and spirit, He chose to work through the physical - healing with mud, instituting bread and wine as His body and blood, and dying upon the Cross. "But that you may believe that the Son of Man hath the power to forgive sins, take up thy bed and walk!"
Think of a relic as a prism or magnifying glass through which the Light shines . . . if that helps, or not if it doesn't.
That, my FRiend, is an excellent explanation. Thank you for enlightening me.