It’s not really a special symbol, it’s actually just a ligature for the Latin word “et”, which means “and”. It’s a bit abstract now but if you squint you can sort of see how it’s just an “e” and a “t” joined together. “Ampersand” actually means “and per se, and” (“and by itself, ‘and’”), meaning the & ligature was sometimes included at the end of the alphabet, by itself, almost like a separate letter.

We don’t really use any in English anymore, except “æ” and “œ”, which are also from Latin (or Greek) and are more obviously two letters (a+e/o+e). German and the Scandinavian languages also use several ligatures - German has ß, which was originally an s+z, and the two dots in ä, ö, and ü were originally an e.

The & ligature is found in Latin documents, and any other language that has the word et, like French. But ancient and medieval documents were written on expensive parchment or paper and sometimes the people who wrote and copied the text wanted to save as much space as possible, so they used a lot of ligatures. There were ligatures for the most common sequences of letters that readers could easily expand…they’re a bit difficult to reproduce on a keyboard, but for example, a shape that looked something like a 9 could replace “con” or “com” or “cum”, while something like a 3 replaced “bus”. An “n” or an “m” could be replaced by a bar over the previous vowel; “per” or “par” could be represented by a “p” with a slash through the bottom part of the stem. The list could go on and on - there are entire books dedicated to abbreviations and ligatures in Latin and other medieval languages. And in fact, short words like but, or, so - they did often have abbreviations/ligatures too, just like “and”. The Latin word “vel” meaning “or” might delete the “e” and have a slash through the “l” (something like “vł”).

“Et” is used so often in Latin that it was sometimes replaced by a symbol that looked kind of like a 7. This originated in “Tironian notes”, a kind of shorthand system that was attributed to Cicero’s slave/secretary Tiro. Tironian notes were almost completely forgotten by the Middle Ages, except for this one symbol for “et”, which even passed into other languages, to be used for their words for “and” even if it wasn’t specifically replacing the word (or sequence of letters) “et”. It’s still used in Irish, at least.

So, basically we have the & symbol in English because it’s actually a ligature of the Latin word “et”, and the word “and” and equivalent words in other languages are so common that we just borrowed one of the pre-existing Latin symbols for it.