Cigars, if not properly stored, can dry out. This can cause the wrapper to crack and/or unravel. Perhaps more importantly, much of the flavor is lost. Some argue you can revive a dried-out cigar, but most agree it will never taste as good as cigar which was properly stored.
Cigar storage is meant to mimic the hot and humid climates where tobacco is grown and cigars are made. Therefore, a temperature around 70°F and 73-74% relative humidity is generally considered ideal. Since these are levels above where we generally keep our homes, a cigar humidor will be necessary for longer-term storage, particularly if you have a large or growing collection. There are humidors that you can purchase, or you can construct your own.