Wireless basic question

I am trying to understand wireless. And I have a few questions.

1. Why did the designers of 802.11 standard decided to use only 20 mhz of bandwidth? Why not use the entire 2.4 ghz band (83.5 mhz) ? Why was it split into channels ??

2. What is the difference between l2 and l3 roaming conceptually ? (I will read through the nitty gritty details from google, but a little explanation in would be really great).

I have never worked on wireless systems and trying to understand the basics. I am sorry if this is a silly question.

Thanks in advance.