If you've invested in a lens it's important to protect it. The last thing you want to do is knock it and end up with a chip or a scratch. Neoprene covers are a great way to prevent this, they provide a soft padded covering for your lens. Wildlife Watching Supplies' cover is made from sections of shock-absorbing 4mm thick water-proof neoprene. Your lens is effectively safe from knocks and scratches. It also offers fairly effective weatherproofing, but not complete - there are small gaps between the neoprene sections. For full weather proofing I suggest their bag cover.
You can choose from a range of different patterns/colours for your cover, mine is in Advantage Timber camouflage, and is reversible to be Olive Green.
In the field, I find it makes it far more comfortable when holding your lens, the soft neoprene is certainly nicer than the metal! And it'll help prevent it overheating or even getting too cold if you're shooting in extreme environments. My only qualm is that it can make operating switches on your lens slightly awkward. By pulling back the section covering them it will make it a lot easier to use.
With lenses that zoom, or have retractable lens hoods, the set will contain a neoprene section with Velcro, so it can be strapped on when the lens is zoomed in or the hood extended, it can then be quickly and easily removed when you are finished.
Whether or not you will definitely benefit from one of these can depend on what sort of photographer you are, if you're going to be out in all terrains and in all weather, then make sure you have one. Even if you aren't though, I would personally advise owning a cover for your lens, just for the peace of mind, and to keep it in top condition!