Many anglers like to use a silicone spray on their fishing line because it helps to improve casting distance, especially when used on braided fishing lines like Spiderwire and Fireline. Another benefit of spraying the line with some sort of lubricant is that it will repel water, making the line more buoyant. While using a silicone spray does indeed improve casting distance by reducing friction between the line and the rod guides, the use of these sprays also have some drawbacks.
The most obvious drawback is scent. While there isn't a substantial amount of research on the ability of fish to detect the scent of silicone sprays, most fishing purists will insist that it certainly will not act as a fish attractant. Another drawback is that it may not be a very environmentally-friendly thing to do. Most lubricating sprays are sold in aerosol cans, and some formulations include things like petroleum, hexane, mineral spirits, and carbon dioxide.
If your goal is to improve casting distance, an effective and environmentally-friendly alternative is to use a graphite-based lubricant. Simply apply powdered graphite to insides of the guides and this will accomplish the same thing as a silicone spray. Not only are powdered graphite lubricants safer for the environment, but they are scentless and odorless. Powdered graphite is also sold in smaller containers which are easier to carry around than a can of spray silicone.
Another alternative is to replace the guides on your fishing rod. If your rod is several years old, there's a very good chance that the inside of the guides are not as smooth as they were when your rod was new. There are many manufacturers who offer replacement guides for your rod for less than a dollar each. Simply replacing worn-out guides is more cost effective than using a spray lubricant on your line.
If your goal is to increase the buoyancy of the line or to repel water from the line, there are dozens of floatants made just for this purpose. Many of these floatants are water soluble and safe for the environment. These are often sold in liquid or paste form, and one extra benefit of using a floatant on your line is that it will stretch out the life of the line and prevent it from getting brittle with age. This is a very important bonus, because the last thing you want to worry about when you have a trophy lunker on the hook is having your line snap because it is brittle.