Excel is used for client-side only programs. There are no servers at all. All programs originally were client-side only originally because networks didn't exist.
A CS grad student friend of mine was in a programming language class, and the instructor was lecturing about visual programming languages, and claimed that there weren't any widely used visual programming languages. (This was in the late 80's, but some people are still under the same impression.)
He raised his hand and pointed out that spreadsheets qualified as visual programming languages, and were pretty darn common.
They're quite visual and popular because of their 2D spatial nature, relative and absolute 2D addressing modes, declarative functions and constraints, visual presentation of live directly manipulatable data, fonts, text attributes, background and foreground colors, lines, patterns, etc. Some even support procedural scripting languages whose statements are written in columns of cells.