Modern taxes pay for many things Military, police, roads, (in the civilised bits) health, schools etc.

What did taxes used to be spent on?

I'm guessing military, but what else (apart from the King having a good time)?

Each tax was raised for a specific purpose and was meant to last a specific time. Usually for wars
Useful guide from HMRC & National arxhives
https://www.nationalarchives.gov.uk/help-with-your-research/research-guides/taxation-before-1689/#:~:text=2.-,Taxes%20in%20the%20medieval%20and%20early%20modern%20periods,Each%20tax%20was%20levied%20separately.

A few things that might also be a taxation.
judges and the legal system
Upkeep of Royal property
Privy councillors
Royal favour
Royal accommodation
Crown estates

As Mel Brooks says "It's good to be the king"