Truing - Making the wheel straight so it doesn't wobble when spun round and also so it is a complete perfect circle.
The bike wheel is pre-stressed. The tight spokes hold everything in place. The striaghter and more perfect it is the stronger it is. So if there is a slight buckle in the wheel it is easier to get a bigger buckle in it.
If you imagine releasing all the spokes then the wheel would obviously fall apart. It is not a major job and should be around £10/wheel max. A loose spoke is just more likely to result in a failed wheel. 1/4 turn with a spoke key can improve it a lot.
By loading the bike up with equipment you are placing more stress on the wheel due to greater weight. Ensuring the wheel is as strong as it can be is just 'good practice'.
Similar 'good practice' could be applied to replacing the brake pads if worn and replacing the brake cables if they are a few seasons old. The extra load may just be too much for them to take. A few pounds spent now on maintenance could save a long push and/or crashed bike. Even if a snapped brake cable doesn't casue harm I prefer to change them in the comfort of my garage rather than at the side of the road, in the dark, when its raining and I'm late.
Couple of good videos on YouTube if you search wheel truing.
FootSore