The Spy Satellite is a utility satellite that provides visibility of a large area around its deployment position. As it takes up very little space on the orbital plane and is cheap, it is an excellent addition to add to the fringes of your orbital plane. These are especially useful with Firaxite coverage bonuses, as you can deploy a satellite over the top of another player's cities if they have Firaxite in their borders. Remember that this is not active visibility but rather just exploration visibility - you aren't able to see units.