Glass rings and fingermarks have nothing to do with sealing as they will just appear.
Sealing (from what I've read) protects the granite from staining for example since it helps protect stuff from seeping into the structure of the stone.
Some granite actually doesn't need to be sealed and one can test this at the granite yard - don't know whether or not this is applicable to your granite.
ETA - There are different colors, patterns and finishes that show less smudging etc. You would probably do better going to a stone yard and getting your materials there. In my experience the selection is better and the cost is lower.
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