What are the dependencies given on "rvm requirements" useful for? - ruby-on-rails

Question

Why does RVM, Ruby or Rails need libc6-dev, ncurses-dev and libtool ?
on Ubuntu running rvm requirements gives us:
Requirements for Linux ( DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=10.10
DISTRIB_CODENAME=maverick
DISTRIB_DESCRIPTION="Ubuntu 10.10" )
NOTE: 'ruby' represents Matz's Ruby Interpreter (MRI) (1.8.X, 1.9.X)
This is the original / standard Ruby Language Interpreter
'ree' represents Ruby Enterprise Edition
'rbx' represents Rubinius
bash >= 4.1 required
curl is required
git is required (>= 1.7 for ruby-head)
patch is required (for 1.8 rubies and some ruby-head's).
To install rbx and/or Ruby 1.9 head (MRI) (eg. 1.9.2-head), then you
must install and use rvm 1.8.7 first.
Additional Dependencies:
# For Ruby / Ruby HEAD (MRI, Rubinius, & REE), install the following:
ruby: /usr/bin/apt-get install build-essential openssl libreadline6
libreadline6-dev curl git-core zlib1g zlib1g-dev libssl-dev
libyaml-dev libsqlite3-dev sqlite3 libxml2-dev libxslt-dev autoconf
libc6-dev ncurses-dev automake libtool bison subversion pkg-config
# For JRuby, install the following:
jruby: /usr/bin/apt-get install curl g++ openjdk-6-jre-headless
jruby-head: /usr/bin/apt-get install ant openjdk-6-jdk
# For IronRuby, install the following:
ironruby: /usr/bin/apt-get install curl mono-2.0-devel
Why does RVM, Ruby or Rails need libc6-dev, ncurses-dev and libtool ?
For the other packages, as far as i can make out:
build-essential automake bison autoconf pkg-config is useful for compiling from source
libreadline6 libreadline6-dev is needed for irb's readline support
openssl libssl-dev is required for ruby's SSL support
curl git-core subversion is required for fetching software
zlib1g zlib1g-dev is required for unpacking rubygems
libyaml-dev is required for ruby's YAML support
libsqlite3-dev sqlite3 is required for sqlite support for rails
libxml2-dev libxslt-dev is required for parsing XML via nokogiri
Want to know why RVM added them in the first place?
What well known libraries will break if any of these packages are not installed?
Which of these packages are necessary for ruby itself to be installed and which of them are needed for a stdlib ie. yaml or rexml ?
EDIT: forgot pkg-config
EDIT: added some more questions at the end

Answer 1

ruby comes with a set of extensions, some of them require additional packages, for example ncurses is needed for better console support (irb) - but is not required, pkg-config is needed for packages searching - most packages expose a configuration file which helps in finding proper paths for compilation ... and so on, here is list of all extensions: https://github.com/ruby/ruby/tree/trunk/ext ... although it's not the list of requirements - i can not find official ruby list of requirements.

Answer 2

libc6-dev are the C standard library development files. It's very unlikely that any useful C code would compile without them. As Ruby is written in C, those are needed to compile and build ruby itself.
I would guess that ncurses is needed to compile irb.
I would agree with the purpose of the other packages you mention.

Answer 3

These are all basic libraries which are dependencies for various gems you will use in future.
build-essential openssl libreadline6
libreadline6-dev curl git-core zlib1g zlib1g-dev libssl-dev
libyaml-dev libsqlite3-dev sqlite3 libxml2-dev libxslt-dev
autoconf libc6-dev ncurses-dev automake libtool bison subversion
There may be a case like you will not require all the libraries in you m/c, though when you will install some gems, errors like
cannot build native extension
In those cases the required libraries are looked upon for the failed gem and installed. Libraries are installed only once and are totally different as compared to gems. The libraries present here are for
Git, mysql server, sqlite, curl etc.
I would suggest to install it in the beginning with sudo permissions using Sudo apt-get install. Hope this helps. Also as you try out new gems and plugins, you would be installing new libraries in future.