Extra Elements
Joao writes:
Sets typically come with a few extra small parts. I always thought
these were random freebies that came along because it was easier for
LEGO to package them with a bit of error. I’ve now seen websites with
part lists where they list the exact quantities of these extra parts,
so the freebies everyone gets seems to be consistent.These parts are not advertised in the pcs count on the box. Why are
these extra parts included with the sets then?Thank you.
Dear Joao,
LEGO weighs each bag in a set during the packaging process. The bags must fall within a weight range or they will be removed from the line. Additional small light weight elements are often included in sets. It must be more efficient and cost effective for TLG to include these additional pieces than manually check the contents of each bag.
Sincerely,
LFan
Ask a LEGO Fan 
An alternate explanation: those are the parts most likely to be lost. TLG includes spares to save themselves the cost of support calls and replacement parts where customers complain that those parts were not included in the package. (Whether that complaint stems from losing the part or not being able to find it doesn’t matter.) It’s just a good business decision.