IMO, people can carry a lot of potential identifiers on them, which could leave behind trace evidence. Pet or stock hair/dander, chemical residues (fertilizers, reagents, cleaning supplies, etc), oils/lubricants/gas, paints/inks/dyes, grain or wood dusts, metal shavings, fabric/threads, various manufacturing residues, and probably a million other things.
I realize that those identifiers alone might not be enough to link somebody specifically to a crime, but if there was something other than DNA left at the crime scene, maybe these kinds of things could possibly help? Just a thought.