Generally, each team has one jungler, which means that there will be one lane of two, and two lanes of one. Since the middle lane will always be a solo lane, this means that top and bottom will have varying numbers of champions, depending on the lane.
Top is usually considered a solo lane because the bottom lane is closer to Dragon, an important mid-game objective. Dragon is hard to solo for most non-junglers, and by keeping 2 champions in the bottom lane, it becomes quite viable to 3-man Dragon after a successful gank (or simply a double-kill). While top lane is closer to Baron Nashor, it takes quite a bit more effort to kill him - more than 3 champions in the early game can probably afford to put out. By the time Baron becomes available, the game is usually nearing the end of the laning phase, after which the difference between top and bottom lane is moot (because all 5 champions are roaming).
With a top solo lane, the champion has a leveling advantage and the potential to farm more minions (simply because there is no allied champion to compete with). Additionally, one must consider that if the other team does not have a jungler, they need to be able to successfully last 2 vs. 1. Most often, this means champions with good sustainability are better suited to the lane, such as Cho'goth, Mordekaiser, Vladimir, or Nasus.
Conversely, there are some champions who do not do well in a solo lane - these tend to be support champions like Soraka, Sona, or Janna who perform much better with a partner to buff/shield/heal. These champions usually go to the bottom lane to ensure that they are in a 2 vs. 2 situation.
In instances where there is no jungler on either team, both top and bottom will have an even number of champions. In this case you may be asked to "switch lanes" not because one lane is "better suited" than the other, but because switching lanes means changing which enemy champions you're fighting (and presumably, have been dying to).
The following forum thread (in date of the Feb-08-2012) states that :
In keeping with this objective, we’re going to begin distributing some rewards to our new players to help them on their way to level 30. Once a new summoner reaches level 3 they will receive 400 Riot Points as a reward for their participation and perseverance up to that point.
See [1]
Other accounts have received these 400 RP in december [2].
Best Answer
The answers already given are correct, but in most cases, players use windowed mode cause they are not at home, and they are used to one resolution. Pro players usually play on one certain resolution, and if they have to go to a LAN event, they tend to set the resolution on their PC to the resolution they are used to, cause even small changes can make a huge diffrence (you cant predict the range exactly if you are used to an other resolution).
Obviouly it also helps with Streaming and/or Alt + Tabbing, but the biggest reason for playing in windowed mode (for pros at least) is the resolution.