I have read tutorials asking you to construct "lit" shelters. What is the benefit of lighting your shelter ?
During the night, monsters can spawn. But they will own spawn in areas that are below a certain "light-level". Most mobs require a light level of 7 or lower to spawn. A torch will emit a light level of 14.
Even if your dwelling is completely safe from things getting in, without a suitable light level, there's nothing stopping the mobs spawning inside with you.
You can figure out which mobs spawn at what light levels, and how much light some blocks emit. You might also need to consider the light radius as well, as while some blocks can emit enough light, it will fade over distance. Therefore one torch in the middle of a 64x64 block house isn't going to work...
Based on the rules of mob spawning, hostile mobs only spawn at light level 7 or smaller. Once you have built your nice shelter, you need to light it up to above this level with torches, glowstone, redstone lamps etc. to avoid the potential for hostile mobs to spawn within your structure and attack you.