The simplest way to look at it is that
a) you need an excess of energy intake relative to your total daily energy expenditure to grow new tissue* - combined with the appropriate resistance training in order for the surplus to actually be used for growth - we all know what happens if you eat for 3 people and don't do any exercise
b) you can physically only build so much lean tissue over a day/week/month - it's like having a team of workers building a house; you can pay them more to work faster, but after a certain point, no amount of money will be able to make them work any quicker - so excess calories beyond a certain point will just end up contributing to an expanding waistline.**
Therefore it's a balance of seeing how much food you can eat whilst weight gain still stays within an acceptable rate.*** For a non-ehanced lifter, muscle gain is a slow process and you want to spend as much time building as you can and as little time having to diet down, so getting fat makes no sense - better to float between 'beach lean' and slightly fluffy vs spending all year trying to get diced then putting on 23kg in 6 months, getting depressed and then spending forever trying to get rid of it.
Read this which will explain it better, especially in terms of why a normal average joe shouldn't emulate what a professional (i.e. PED-using) bodybuilder should be doing.
http://www.bodyrecomposition.com/muscle-gain/general-philosophies-of-muscle-mass-gain.html
*for the obese with huge energy availability (from their excess of fat) and newbie lifters, muscle gain on a calorie deficit is possible - the leaner you get and the less inexperienced you are, the harder this becomes.
**the amount of a surplus you can get away with will be determined various things (some people just have better genetics for nutrient partitioning, a teen full of hormones and still growing vs. someone middle-aged, the closer you are to your genetic potential the slower the rate of growth, so a surplus may have to be tiny etc), so there isn't a blanket answer for how much you should over-eat, although WHAT you should be eating is a different matter and more easily answerable.
***usually this can be something like 2-3lbs a month for newbie male lifters, 1-2lbs for intermediates and 0-1lbs for more advanced males (females it gets halved).