I don't know why they sell them together, but I bought a box once that had a tie, pocket square, and a pair of cufflinks all the same material. I don't wear them together, but I like the pattern and use all three pieces.
All of my pocket squares are silk as well as my ties. There is no issue with wearing a silk square and a silk tie. You could also do cotton or linen though.
All of my pocket squares are silk as well as my ties. There is no issue with wearing a silk square and a silk tie. You could also do cotton or linen though.