Personally I learned from doing stage lighting at school, but there is plenty of free information available online if you look for it.
If you look at
this site under the section for 'beam angles' it gives an illustration of how using bulbs with wider beam angles will only give the same intensity on the floor when they are overlapped. This might seem like using lights with narrow beam angles is a simpler option to achieve the same effect, but it's also important to consider shadows - you're not just trying to get as much light into a room as possible, you're trying to eliminate shadows. Notice in the picture how with the narrow beam angles the room is only lit fully when the light gets to the floor - anything above there, say a table, and if you're not directly under a light you'll be in the dark, and when you are under a light you'll have a bright spot of light on the top of your head, like you're being 'beamed up' or you're in the video for Bohemian Rhapsody. This is because a single point of light will only light a single dimension, making things look flat. If you want to light something up 3 dimensionally you'll need to light from different angles, such as overlapping the beams or using a softer source like a lampshade or light reflected from a surface.
Think about the definition of the beam - if you've got a sharply defined beam of light, say from an MR16, you'll also get a sharply defined shadow. This famously makes jewellery 'sparkle', but not every lighting situation is lighting a jewellery shop - using the same lights in a room may well just create bright circles of light on the floor while the rest of the room appears dark.
Consider a kitchen, with work surfaces around the outside. A fluorescent strip light in the middle may not look elegant but the light from it will reflect off the cabinets and all around the room. Replacing a central fluorescent strip with downlighters might look more 'modern' but, as in the picture on the website, it'll be lighting the floor in the middle instead of the worktops. Angle the spots outwards towards the worktops and when you're standing at the worktop you'll have a perfectly cast, crisp shadow right over what you're trying to work on. In this situation if you can't have the spots directly over the worktops (this will still cast shadows underneath what you're working on and make it appear one dimensional so it's not ideal) it would be better to use lights under the cabinets.
In response to the example in this thread it was mentioned that angling the spots into the room may result in the lights shining in people's eyes.
Consider a situation where you want to see who someone is in the dark. Shining a torch in their face will light it up really well so the person holding the torch can make out who they are, but from their point of view they'll be dazzled so won't be able to see anything. This illustrates how you need to consider where the light is coming from, while at the same time considering anything that might get in the way to cast a shadow.
As you can see from this, downlighters aren't a one size fits all solution and can cause a lot of problems. If you're not careful you can end up with a room which looks like it's being lit with a load of torches in the ceiling.
