Why did visual terminal VT100 need a fixed-width font? There were visual limits: 24 x 80 characters. But still why not use variable-width characters?For instance, in PuTTY documentation in paragraph 4.8.2 it is said that variable-width font characters are redrawn so that they fit one fixed-width font character.
Why do VT100 terminals use only fixed-width fonts?
terminal;hardware
VT100s predate (1976) any use of variable-pitch fonts in terminals.  That is incidental.  There is more than one reason for using fixed-pitch fonts in terminals, but the main one is that cursor-addressing assumes a fixed-pitch layout of characters (a grid of rows/columns).With proportional fonts, the characters are different widths, and do not fit into a regular grid (letter i is narrower than M or W for example).Proportional fonts in terminals did not show up until the 1980s, and then rarely as in 9term.  With those, you cannot use programs such as vi (or vim), but instead editors such as sam.PuTTY's documentation says, in 4.8.2:By default, you will be offered a choice from all the fixed-width fonts installed on the system, since VT100-style terminal handling expects a fixed-width font. If you tick the box marked Allow selection of variable-pitch fonts, however, PuTTY will offer variable-width fonts as well: if you select one of these, the font will be coerced into fixed-size character cells, which will probably not look very good (but can work OK with some fonts).Whether the font is scalable or not, individual glyphs are not, without a lot of work.  The result will show either lots of empty space, or characters which overwrite their neighbors when displayed on a fixed-pitch grid.Initially xterm did not allow proportional fonts but (in 1998, before PuTTY) it was modified to permit this with the caveat that the result would not be good.  The font tells the minimum and maximum size of characters without any hint where the majority of characters lie on that range.  xterm's forcePackedFont resource allow you to choose, which extreme to use.