Well 'ed' (a line-based editor) and 'vi' (the screen-based editor) were for a long time the least common dominater when it came to editors on Unix-systems... sure they may also have had other better editors like emacs installed, but *at least* they would have 'ed' and perhaps 'vi' available if they needed to edit a file. These days -- at least for Linux -- they're being replaced with 'nano' on rescue-cds and such...
Emacs is pretty big, so way back when, it would simply take up too much room on a rescue-*floppy*... much better to use a smaller editor like 'ed' or 'vi'. Also, if you were working in single-user mode without any additional partitions mounted yet (perhaps your editing fstab and /usr is on seperate partition... maybe even mounted over the network), the only programs guaranteed available to you, would be those in /bin and /sbin -- no /usr/bin (ie. 'emacs') and certainly no X11... Remember many system started in single-user mode, and the administrator had explicitly bring it up fully. It was assumed that after a reboot, the administrator would have to clean-up after a crash or do other maintanace.
If you're working on a text-terminal (as one often did), then X (and thus editors like 'kate') would be out of the question anyway.
'ed' and 'vi' are utilities that were "part of" the operating-system itself (and therefor in /bin rather than /usr/bin)... 'emacs' and 'kate' are optional applications.
I would like to say a few word about the "original" editor, 'ed'... This is a purely linebased editor, where you have no arrow-keys or the likes to move up or down a line. It came from the time when one used tele-typewriters (telex-machines) -- typewriters and a roll of paper -- to log-in to a system. Here you could't move the cursor up, down, right or left; you could only type-out line after line. So 'ed' does just that -- do things like type-out lines 5 through 10 of a file.
Why is this useful today? Sometimes the terminal you work on is unknown or doesn't get identified... or maybe the list over terminal-types on your system has been broken? If that is the case, any program relying on moving the cursor -- like most editors, including 'vi' -- won't work! The only thing you can safely do, is to print-out line following line, moving only in one direction.
Originally 'vi' was actually a wrapper around another line-based editor (similar to 'ed') called 'ex'. Today we use vi-clones like 'elvis' and ViImproved ('vim'), and 'ex' and 'vi' are just links to one of those...
'vi' vs. 'emacs' is almost an religious dispute among some Unix-users -- the simple versus the the complex. Of course 'vi' is somewhat obscure and difficult to use (though so is 'emacs'), but when you first learn it, 'vi' is a *great* editor... especially for doing typical "system-admin-stuff", like editing various configuration-files. Unlike some editors, 'vi' doesn't start breaking lines and inserting lineshifts everywhere...
My advice, learn 'vi' (or 'vim') and 'ed' both!