The Spy has 3 main goals.
1. Harass Engineers.
2. Disrupt the Heavy-Medic Pair.
3. Dispatch Snipers.
Through this activity he succeeds at.
1. Collecting Recon
2. Disrupting the Enemy.
3. Instilling Paranoia.
A good spy has done his job right if they enemy is over compensating for him, instead of concentrating on the rest of the team.