Taxation is a nationwide contribution to the government's operating budget from every person willing and capable of legally earning money.
Tax money pays for public services like police, firemen, health care, justice, garbage disposal, education, road maintenance and so on. It's a long list.
No reason why they cant be private businesses instead. Government is unneeded and free market can take care of that stuff.
Whether or not it is needed is irrelevant. Taxation isnt theft as you do receive legitimate services in return. It can be considered extortion since theres threat of force if you refuse to pay.