Alchemical recipes are the primary function. The ingredients, products, stages, planets, etc. all have a symbol.

Sorry, I'll have to update slowly throughout the day.

What you are basically trying to do as an alchemist is change the aspects or features of your starting matter to different aspect until it becomes that different matter. Like change lead to gold.

Basically changing the proportions of air, earth, fire, and water in matter. Each of those elements have a symbol. A triangle, with earth and air having a horizontal line through it.

Then the products and some starting ingredients have a symbol. like Aqua Regia is the symbol for water (a triangle pointing down) with the letter "R" stuck to the right side.

Another example, salt's symbol is the greek letter Θ ...so then some salt compounds' symbols are variations of that symbol.

In addition you have symbols for measurements, like half ounce, half dram, etc.

So why?

There is a long tradition of seeing this sort of knowledge as potentially dangerous, or not meant for everyone. So one part was keeping it secret and using symbols that only the initiated can read. Even if you could read it, if it failed, that meant God didn't want you to know. The theories vary, but some tie esoteric knowledge back to divine knowledge.. so protect it from heathens (basically) others were more worried about what would happen if gold became commonplace. Kings (or the church) would outlaw alchemy and to protect the economy, but then hire their own court alchemists.

So if you weren't a court alchemist, having recipes in plain text could actually be dangerous because it was illegal.

(possibly) more later. I realize I'm missing sources, my books aren't here. This is pretty basic alchemical understanding though... the best I could do is say which History of Alchemy podcast episode I covered this on (but that's just sourcing myself anyway)